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Copyright (C) The Internet Society (2004). All Rights Reserved.
This document specifies the URI (Uniform Resource Identifier) scheme "tel". The ``tel'' URI describes resources identified by telephone numbers.
3. URI Syntax
4. URI Comparisons
5. Phone Numbers and Their Context
5.1 Phone Numbers
5.1.1 Separators in Phone Numbers
5.1.2 Alphabetic Characters Corresponding to Digits
5.1.3 Alphabetic, * and \\# Characters as Identifiers
5.1.4 Global Numbers
5.1.5 Local Numbers
5.2 ISDN Subaddresses
5.4 Other Parameters
7.1 Why Not Just Put Telephone Numbers in SIP URIs?
7.2 Why Not Distinguish Between Call Types?
7.3 Why tel?
7.4 Do Not Confuse Numbers with How They Are Dialed
8. Usage of Telephone URIs in HTML
9. Use of tel URIs with SIP (Informative)
9.1 Local Translation
9.2 Proxy Translation
10. IANA Considerations
12. Security Considerations
13. Change History
13.1 Changes from ietf-02 to ietf-03
13.2 Changes from ietf-00 to ietf-01
13.3 Changes from -08 to draft-ietf-iptel-rfc2806bis-00
13.4 Changes Since -07
13.5 Changes Since -06
13.6 Changes Since -05
13.7 Changes Since -04
13.8 Changes Since -03
13.9 Changes Since -02
13.10 Changes Since -01
13.11 Changes Since RFC 2806
§ Normative References
§ Informative References
§ Author's Address
§ Intellectual Property and Copyright Statements
This document defines the URI scheme "tel". The "tel" URI describes resources identified by telephone numbers. A telephone number is a string of decimal digits that uniquely indicates the network termination point. The number contains the information necessary to route the call to this termination point. (This definition is derived from [E.164], but encompasses both public and private numbers.)
The "tel" URI telephone number is not restricted in the type of termination point it refers to. The termination point can be in the public telephone network, a private telephone network or the Internet. The termination point can be fixed or wireless and address a fixed wired, mobile or nomadic terminal. The terminal addressed can support any electronic communication service (ECS) including voice, data and fax. The URI can refer to resources identified by a telephone number, including but not limited to originators or targets of a telephone call.
The "tel" URI is a globally unique identifier ("name") only; it does not describe the steps necessary to reach a particular number and does not imply dialing semantics. Furthermore, it does not refer to a specific physical device, only to a telephone number.
Telephone numbers as commonly understood actually comprise two related, but distinct concepts: as a canonical address-of-record and as a dial-string. We define the concepts below:
- Address-of-record or identifier:
- The telephone number is understood here as the canonical address-of-record or identifier for a termination point within a specific network. For the public network, these numbers follow the rules in E.164[E.164], while private numbers follow the rules of the owner of the private numbering plan. Subscribers publish such identifiers phone number as a universal means of being reached, independent of the location of the caller. (Naturally, not all numbers are reachable from everywhere, for a variety of technical and local policy reasons. Also, a single termination point may be reachable from different networks and may have multiple such identifiers.)
- Dial string:
- "Dial strings" are the actual numbers, symbols and pauses entered by a user to place a phone call. A dial-string is consumed by one or more network entities, and understood in the context of the configuration of these entities. It is used to generate a telephone number so that a call can be routed. Dial-strings may require pre-pended digits to handle local PBXs, and they may include post-dial DTMF signaling that could control an IVR or reach an extension. Dial strings are beyond the scope of this document.
To reach a telephone number from a phone on a PBX, for example, the user of that phone has to know how to convert the telephone number identifier into a dial string appropriate for that phone. The telephone number itself does not convey what needs to be done for a particular terminal. Instructions may include dialing "9" before placing a call or prepending a "00" to reach a number in a foreign country. The phone may also need to strip area and country codes.
The notation for phone numbers in this document is similar to that in RFC 3191[RFC3191] and RFC 3192[RFC3192]. However, the syntax differs since this document describes URIs whereas RFC 3191 and RFC 3192 specify electronic mail addresses. RFC 3191 and RFC 3192 use "/" to indicate parameters (qualifiers). Since URI use the forward slash to describe path hierarchy, the URI scheme described here uses the semicolon, in keeping with Session Initiation Protocol (SIP) URI conventions [RFC3261].
There are at least two ways one can envision making a telephone connection. In the first approach, a URI contains the dial string, which is then passed to an entity that can reproduce the actions specified in the dial string. For example, in an analog phone system, a dialer translates the dial string into a sequence of actions such as waiting for dial tone, sending DTMF digits, pausing and generating post-dial DTMF digits after the callee picks up. In an ISDN or ISUP environment, the recipient of the dial string performs the appropriate protocol actions.
Another approach has the URI specify the telephone number, which can be either globally unique or only be valid within a local context. The dialing application is aware of the local context, knowing, for example, whether special digits need to be dialed to seize an outside line, whether network, pulse or tone dialing is needed and what tones indicate call progress. The dialing application then converts the telephone number into a dial sequence and performs the necessary signaling actions. The document below assumes the second model. The dialer does not have to be a user application as found in traditional desktop operating systems, but could well be part of an IP-to-PSTN gateway.
The approach pursued here has the disadvantage that certain services, such as electronic banking or voicemail, cannot be specified in a URI.
The URI can be used as a request URI in SIP [RFC3261] requests. The SIP specification also inherits the 'subscriber' part of the syntax as part of the 'user element' in the SIP URI. Other protocols may use this URI for both query-based and prefix-based applications.
The "tel" URI does not specify the call type such as voice, fax, or data call and does not provide the connection parameters for a data call. The type and parameters are assumed to be negotiated either in-band by the telephone device or through a signaling protocol such as SIP.
In this document, the key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" are to be interpreted as described in BCP 14, RFC 2119[RFC2119] and indicate requirement levels for compliant implementations.
The URI is defined using the ABNF (augmented Backus-Naur form) described in RFC 2234[RFC2234] and uses elements from the core definitions (Appendix A of RFC 2234).
The syntax definition follows RFC 2396[RFC2396], indicating the actual characters contained in the URI. Note that the reserved characters "+", ";", "=", and "?" MUST NOT be escaped if shown in the grammar definitions below as they are delimiters for the "tel" URI scheme. These reserved characters MUST be escaped if they appear in parameter values.
Characters other than those in the "reserved" and "unsafe" sets (see RFC 2396[RFC2396]) are equivalent to their "% HEX HEX" encoding.
The "tel" URI has the following syntax:
telephone-uri = "tel:" telephone-subscriber telephone-subscriber = global-number / local-number global-number = global-number-digits *par local-number = local-number-digits *par context *par par = parameter / extension / isdn-subaddress isdn-subaddress = ";isub=" 1*uric extension = ";ext=" 1*phonedigit context = ;phone-context=" descriptor descriptor = domainname / global-number-digits global-number-digits = "+" 1*phonedigit local-number-digits = 1*phonedigit-hex domainname = *( domainlabel "." ) toplabel [ "." ] domainlabel = alphanum / alphanum *( alphanum / "-" ) alphanum toplabel = ALPHA / ALPHA *( alphanum / "-" ) alphanum parameter = ";" pname ["=" pvalue ] pname = 1*( alphanum / "-" ) pvalue = 1*paramchar paramchar = param-unreserved / unreserved / escaped unreserved = alphanum / mark mark = "-" | "_" | "." | "!" | "~" | "*" | "'" | "(" | ")" escaped = "%" HEXDIG HEXDIG param-unreserved = "[" / "]" / "/" / ":" / "&" / "+" / "$" phonedigit = DIGIT / [ visual-separator ] phonedigit-hex = HEXDIG / "*" / "#" / [ visual-separator ] visual-separator = "-" / "." / "(" / ")" alphanum = ALPHA / DIGIT
Each parameter name ("pname"), the ISDN subaddress, the extension and the context MUST NOT appear more than once. The order of the URL parameters is immaterial. The ISDN subaddress or extension SHOULD appear first, if present, followed by the context parameter, if present, followed by any other parameters in lexicographical order.
This simplifies comparison when the "tel" URI is compared character-by-character, such as in SIP URIs [RFC3261].
Two "tel" URIs are equivalent according to the following rules:
All parameter names and values SHOULD use lower-case characters since tel URIs may be used within contexts where comparisons are case-sensitive.
Section 19.1.4 in the SIP specification [RFC3261] discusses one such case.
The 'subscriber part of the URI indicates the number. The phone number can be represented in either global (E.164) or local notation. All phone numbers MUST use the global form unless they cannot be represented as such. Numbers from private numbering plans, emergency ("911", "112") and some directory assistance numbers (e.g., "411") and other "service codes" (numbers of the form N11 in the United States) cannot be represented in global (E.164) form, and need to be represented as a local number with a context. Local numbers MUST be tagged with a 'phone-context' (Section 5.1.5).
Implementations MUST NOT assume that telephone numbers have a maximum, minimum or fixed length, or that they always begin with a certain number.
E.164 limits numbers to 15 digits. For geographic numbers, one to three digits are the country code, with the remainder divided into area or city code (national destination code) and subscriber number. Alternatively, there is a global three-digit service code, followed by a global subscriber number of up to 12 digits. Finally, a "international public telecommunication number for networks is composed of decimal digits arranged in three code fields. The code fields are the 3-digit shared Country Code (CC) field, the IC field, which varies in length between 1 to 4 digits, and the Subscriber Number (SN) which can be up to 15 minus the number of digits in the CC and IC fields." [E.164].
Phone numbers MAY contain visual separators. Visual separators ('visual-separator') merely aid readability and are not used for URI comparison or placing a call.
Despite complicating comparisons, this specification retains the visual separators to follow the spirit of RFC 2396[RFC2396], which remarks that "A URI often needs to be remembered by people, and it is easier for people to remember a URI when it consists of meaningful components." Also, ISBN URNs documented in RFC 3187[RFC3187] use visual separators in a manner similar to this specification.
Even though ITU-T E.123[E.123] recommends the use of space characters as visual separators in printed telephone numbers, "tel" URIs cannot use spaces to avoid excessive escaping.
In some countries, it is popular to write phone numbers using alphabetic characters which correspond to certain numbers on the telephone keypad. The URI format does not support this notation since the mapping from alphabetic characters to digits is not completely uniform internationally, although there are standards [E.161][T1.703] addressing this issue.
Since called and calling terminal numbers (TNs) are encoded in BCD in ISUP, six additional values per digit can be encoded, sometimes represented as the hexadecimal characters A through F. Similarly, DTMF allows for the encoding of the symbols *, \# and A through D. However, in accordance with E.164, they may not be included in global numbers. Their use in local numbers is not defined, but is not prohibited.
Globally unique numbers are identified by the leading "+" character. Global numbers MUST be composed with the country (CC) and national (NSN) numbers as specified in E.123[E.123] and E.164[E.164]. Globally unique numbers have the property of being unambiguous everywhere in the world and are RECOMMENDED.
Local numbers are unique only within a certain geographical area or a certain part of the telephone network, e.g., a private branch exchange (PBX), a state or province, a particular local exchange carrier or a particular country. URIs with local phone numbers should only appear in environments where all local entities can successfully set up the call by passing the number to the dialing software. Digits needed for accessing an outside line, for example, are not included in local numbers. Local numbers SHOULD NOT be used unless there is no way to represent the number as a global number.
Local numbers require that the originator and recipient are configured appropriately, so that they can insert and recognize the correct descriptors. Since there is no algorithm to independently pick the same descriptor, labeling numbers with their context increases the chances of mis-configuration, so that valid identifiers are rejected by mistake. The algorithm to select descriptors was chosen that accidental collisions should be rare, but they cannot be ruled out.
Local numbers MUST have a 'phone-context' parameter that identifies the scope of their validity. The parameter MUST be chosen to unambiguously identify the local context within which the number is unique. Thus, the combination of the descriptor in the 'phone-context' parameter and local number is again globally unique. The parameter value is defined by the assignee of the local number. It does NOT indicate a prefix that turns the local number into a global (E.164) number.
There are two ways to label the context: via a global number or any number of its leading digits (e.g., "+33") and via a domain name, e.g., "houston.example.com". The choice between the two is left to the "owner" of the local number and is governed by whether there is a global number or domain name that is a valid identifier for a particular local number.
The domain name does not have to resolve to any actual host, but MUST"> be under the administrative control of the entity managing the local phone context.
A global number context consists of the initial digits of a valid global number. All global numbers matching these initial digits must be assigned to the same organization that is describing the context and no such matching number can be used by any other organization. If such an initial string of digits does not exist, the organization should use the lowest number of the global number range assigned to it. (This can occur if two organizations share the same decimal block of numbers. For example, assume an organization owns the number range +1-212-939-7000 through +1-212-939-7199. +1-212-939-7 would not be a valid global number context, but +1-212-939-7000 would work.) It is not required that local numbers within the context actually begin with the chosen set of initial numbers.
For a local number defined within a PBX, the organization can choose any number under its control to identify the context. For example, a context consisting of any of the organization's global numbers may be suitable, or a substring that is completely occupied by the organization. For example, +49-6151-16 would be a suitable context for the TU Darmstadt, as it uses all numbers starting with those digits.
A context consisting of the initial digits of a global number does not imply that adding these to the local number will generate a valid E.164 number. It might do so by coincidence, but this cannot be relied upon. (For example, "911" should be labeled with the context "+1", but "+1-911" is not a valid E.164 number.)
National freephone numbers do not need a context, even though they are not necessarily reachable from outside a particular country code or numbering plan. Recall that "tel" URIs are identifiers; it is sufficient that a global number is unique, but it is not required that it be reachable from everywhere.
Even non-freephone numbers may be out of date or not be reachable from a particular location. For example, premium services such as "900" numbers in the North American numbering plan are often not dialable from outside the particular country code.
The two label types were chosen so that, in almost all cases, a local administrator can pick an identifier that is reasonably descriptive and does not require a new IANA-managed assigned number. It is up to the administrator to assign an appropriate identifier and to use it consistently. Often, an organization can choose among several different identifiers.
If the recipient of a "tel" URI uses the URI simply for identification, the receiver does not need to know anything about the context descriptor. It simply treats it as one part of a globally unique identifier, with the other being the local number. If a recipient of the URI intends to place a call to the local number, it MUST verify that it is within the same context as one of the descriptors. If it is not within the same context, it MUST NOT initiate the call and treat the URI like an invalid destination.
A phone number MAY also contain an 'isdn-subaddress"> parameter which indicates an ISDN subaddress.
ISDN subaddresses typically contain IA5 characters, but may contain any octet value.
Extensions identify stations behind a PBX and are roughly equivalent to ISDN subaddresses. They are identified with the 'extension"> parameter. At most one of the 'isdn-subaddress and 'extension parameters can appear in a tel URI, i.e., they cannot appear both at the same time.
Future extensions to this URI scheme may add other parameters ('parameter in the ABNF). Such parameters can be either mandatory or optional. Mandatory parameters start with "m-". An implementation MAY ignore optional parameters. An implementation MUST NOT use the URI if it contains unknown mandatory parameters. The "m-" prefix cannot be added to parameters that were already registered (except to create a new, logically distinct parameter). The "phone-context" parameter in this document is mandatory.
For example, 'parameter' parameters can be used to store application-specific additional data about the phone number, its intended use, or any conversions that have been applied to the number.
All new parameters MUST be registered with IANA.
- This URI points to a phone number in the United States. The hyphens are included to make the number more human-readable; they separate country, area codes and subscriber number.
- The URI describes a local phone number valid within the context "cs.columbia.edu".
- The URI describes a local phone number that is valid within a particular phone prefix.
The "tel" URI describes a service, reaching a telephone number, that is independent of the means of doing so, be it via a SIP-to-PSTN gateway, a direct SIP call via ENUM translation, some other signaling protocols such as H.323 or a traditional circuit-switched call initiated on the client side via, say, TAPI. It is thus, in spirit, closer to the URN schemes that also leave the resolution to an external mechanism. The same "tel" URI may get translated to any number of other URIs in the process of setting up the call.
Signaling protocols such as SIP allow to negotiate the call type and parameters, making the very basic indication within the URL scheme moot. Also, since the call type can change frequently, any such indication in a URI is likely to be out of date. If such designation is desired for a device that directly places calls without a signaling protocol such as SIP, mechanisms such as the "type" attribute for the "A" element in HTML may be more appropriate.
"Tel" was chosen since it is widely recognized none of the other suggestions appeared appropriate. "Callto" was discarded since URI schemes locate a resource and do not specify an action to be taken. "Telephone" and "phone" were considered too long and not as internationally recognized.
As an example, the E.164 number "+1-212-555-3141" will be dialed in many countries as 00-1-212-555-3141, where the leading "00" is a prefix for international calls. (In general, "+" in E.164 indicates that an international prefix is required.) Tel URIs MUST NOT contain the local dialing prefixes in numbers such as +1-212-555-3141, as the transformation back to an international number is not guaranteed to be correct or unique.
If a network entity receives a "tel" URI containing a local number, it MUST make sure that it knows the context in which the local phone number is to be processed, or else the number MUST NOT be used. Equally, the originator of a "tel" URI must take into consideration that the recipient may have insufficient information about the phone number's context.
Links using the "tel" URL SHOULD enclose the telephone number, so that users can easily predict the action taken when following the link.
Dial <a href="tel:+1-212-555-0101">+1-212-555-0101</a> for assistance.
Dial <a href="tel:+1-212-555-0101">this number</a> for assistance.
On a public HTML page, the telephone number in the URI SHOULD always be in the global form, even if the text of the link uses some local format.
Telephone (if dialing in the United States): <a href="tel:+1-201-555-0111">(201) 555-0111</a>
For having RFCs read aloud, call <a href="tel:+1-555-438-3732">1-555-IETF-RFC</a>.
SIP can use the "tel" URI as a Request-URI, along with "sip" and "sips" URIs. For brevity, we will imply "sips" URIs when talking about SIP URIs. Both "tel" and SIP URIs can contain telephone numbers. In SIP URIs, they appear as the user part, i.e., before the @ symbol (Section 19.1.6 in [RFC3261]).
Unless a SIP UA connects directly to a PSTN gateway, one of the SIP proxy servers has to translate the tel URI to a SIP URI, with the host part of that URI pointing to a gateway. Typically, the outbound proxy server, as the first proxy server visited by a call request, performs this translation. A proxy server can translate all tel URIs to the same SIP host name or select a different gateway for different tel prefixes, based, for example, on information learned from TRIP. However, a proxy server could also delegate this translation task to any other proxy server since proxy servers are free to apply whatever routing logic they desire.
As noted earlier, all phone numbers MUST use the global form unless they cannot be represented as such. If the local-number format is used, it MUST be qualified by the 'phone-context' parameter. Effectively, the combination of local number and phone context makes the tel URI globally unique.
While web pages, vCard business cards, address books and directories can easily contain global tel URIs, users on twelve-button (IP) phones cannot dial such numbers directly and are typically accustomed to dialing shorter strings, e.g., for PBX extensions or local numbers. These so-called dial-strings (Section 1) are not directly represented by tel URIs, as noted. We refer to the translation of dial strings into SIP and tel URIs, global or local, as the dial plan. There are at least two appropriate ways to deal with dial strings in SIP terminals, local translation and proxy translation, described in turn below.
A SIP UA can use a dial plan to translate dial strings into SIP or "tel" URIs. The dial plan can be manually configured or, preferably, be downloaded as part of a device configuration mechanism. (At this time, there is no standardized mechanism for this.)
A mobile user can use at least two dial plans, namely the dial plan for the network that he is currently visiting and the dial plan for the user's home network. Generally, dialed numbers that are meant to represent global numbers will look the same after the translation regardless of the dial plan, even if, say, the visited network uses '0' for dialing an 'outside' number and the user's home network uses '9', as long as the user is aware of the current dial plan. However, local extensions that do not have a direct global equivalent may well behave differently. To avoid any ambiguity, the dial plan MUST insert a suitable 'phone-context' string when performing the translation. If the 'phone-context' is a domain name, there are three cases:
tel:1234;phone-context=munich.example.comto the SIP proxy located at munich.example.com. (Proxies that receive a tel URI with a context they do not understand are obligated to return a 404 (Not Found) status resonse, so that an outbound proxy may decide to attempt such a heuristic.)
In proxy translation mode, the SIP UA simply turns the dialed digits into the user part of the SIP URI and appends a ';user=phone' parameter and provides an appropriate phone context reflecting the local dialing plan. The host name or IP address of the outbound proxy is made the host part of the SIP request URI. The outbound proxy can then apply the dial plan indicated by the phone context in the URI to translate the SIP URI into a "tel" URI or other SIP URI. Translation into a "tel" URI makes sense only if the proxy wants to delegate finding the PSTN gateway to another proxy. For example, after the user with a location-specified dial plan located in domain "munich.example.com" dials the digits "1234", the device converts this into a SIP URI:
Alternatively, the SIP UA can issue a call with a "tel" URI. For this example, it might be:
Using a SIP URI is more robust and is thus the preferred approach.
Since a single proxy may receive calls from many different locations with different local dial plans, devices that rely on the proxy for number translation SHOULD always be configured with a context. Otherwise, for example, a provider or enterprise would have to provision a separate proxy for each branch office or geographic area with its own dial plan.
IANA is requested to update the reference to RFC 2806 in the URI scheme registry for the 'tel' scheme to this document.
"Tel" URI parameters ('parameter') MUST be registered with IANA. Mandatory parameters must be described in a standards-track RFC, while an informational RFC is sufficient for other parameters.
The registration must indicate:
- Parameter name:
- The name used for the parameter, according to the BNF in Section 3.
- A brief description of its applicability.
- Whether the parameter is mandatory or not; only the names of mandatory parameters must start with "m-" as described in Section 5.4.;
- A reference to the specification that defines the parameter and its syntax.
This document is derived from RFC 2806 [RFC2806], written by Antti Vähä-Sipilä. Flemming Andreasen, Francois Audet, Lawrence Conroy, Cullen Jennings, Andrew Main, Michael Hammer, Jon Peterson, Mike Pierce, Jonathan Rosenberg and James Yu provided extensive comments.
The security considerations parallel those for the mailto URL [RFC2368].
A recipient of a "tel" URI SHOULD NOT place calls without the consent of its owner. Placing calls automatically without appropriate user confirmation may incur a number of risks, such as those described below.
The draft has been greatly simplified to reflect parts that have actually been implemented.
The specification is backwards-compatible with RFC 2806.
|[E.123]||, ITU., "Notation for national and international telephone numbers, e-mail addresses and web addresses", Recommendation E.123, February 2001.|
|[E.161]||, ITU., "Arrangement of digits, letters and symbols on telephones and other devices that can be used for gaining access to a telephone network", Recommendation E.161, May 1995.|
|[E.164]||, ITU., "The international public telecommunication numbering plan", Recommendation E.164, May 1997.|
|[RFC2119]||Bradner, S., "Key words for use in RFCs to Indicate Requirement Levels", BCP 14, RFC 2119, March 1997 (HTML, XML).|
|[RFC2234]||Crocker, D. and P. Overell, "Augmented BNF for Syntax Specifications: ABNF", RFC 2234, November 1997.|
|[RFC3191]||Allocchio, C., "Minimal GSTN address format in Internet Mail", RFC 3191, October 2001.|
|[RFC3192]||Allocchio, C., "Minimal FAX address format in Internet Mail", RFC 3192, October 2001.|
|[RFC3261]||Rosenberg, J., Schulzrinne, H., Camarillo, G., Johnston, A., Peterson, J., Sparks, R., Handley, M. and E. Schooler, "SIP: Session Initiation Protocol", RFC 3261, June 2002.|
|[T1.703]||, ANSI., "Allocation of Letters to the Keys of Numeric Keypads for Telecommunications", Standard T1.703-1995 (R1999), 1999.|
|[I-D.yu-tel-url]||Yu, J., "New Parameters for the 'tel' URL to Support Number Portability and Freephone Service", draft-yu-tel-url-08 (work in progress), November 2003.|
|[RFC2368]||Hoffman, P., Masinter, L. and J. Zawinski, "The mailto URL scheme", RFC 2368, July 1998 (HTML, XML).|
|[RFC2396]||Berners-Lee, T., Fielding, R. and L. Masinter, "Uniform Resource Identifiers (URI): Generic Syntax", RFC 2396, August 1998 (HTML, XML).|
|[RFC2806]||Vaha-Sipila, A., "URLs for Telephone Calls", RFC 2806, April 2000.|
|[RFC3187]||Hakala, J. and H. Walravens, "Using International Standard Book Numbers as Uniform Resource Names", RFC 3187, October 2001.|
|Department of Computer Science|
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|New York, NY 10027|
|Phone:||+1 212 939 7042|
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