In 3GPP when the phone recognises that it is an emergency call, and needs to identify the type of emergency call, then it has access to signal categories with the following names (ref 3GPP TS 22.101): ------------------------------ - Police - Ambulance - Fire Brigade - Marine Guard - Mountain Rescue - Spare, at least [three] different types ----------------------------- which results in a coding accompanying the call establishment of (ref 3GPP TS 24.008 table 10.5.135d) -------------------------- Emergency Service Category Value (octet 3) The meaning of the Emergency Category Value is derived from the following settings (Please see 3GPP TS 22.101 clause 8): Bit 1 Police Bit 2 Ambulance Bit 3 Fire Brigade Bit 4 Marine Guard Bit 5 Mountain Rescue Bits 6,7,8 are spare and set to "0" Mobile station may set one or more bits to "1" If more than one bit is set to "1", routing to a combined Emergency centre (e.g. ambulance and fire brigade in Japan) is required. If the MSC can not match the received service category to any of the emergency centres, it shall route the call to an operator defined default emergency centre. If no bit is set to "1", the MSC shall route the Emergency call to an operator defined default emergency centre ------------------------- These terms are at least accepted by multiple SDOs in the international community for transposition into their national standards. I also note that 3GPP does have the capability of signalling multiple preferences - this left me wondering whether we would be better off trying to follow that, possibly with a single emergency uri, but with the possibility of signalling category as a URI parameter with a list of values. Depending on which 3GPP release you get there are a number of mechanisms. 1) Certain numbers are built into the phone and recognised there. 2) Certain numbers are preloaded onto the SIM/UICC card by the operator at subscription time. 3) Numbers can be downloaded to the phone at registration time from the local providing access network. These are identified in the various versions of 22.101 (see http://www.3gpp.org/ftp/Specs/html-info/22101.htm) clause 10. I am not sure what terminal manufacturers do in the case of setting the bits for phones supplied for use in the US, however inspection of the 3GPP TS 24.008 text says that in the US, whatever the bits are set to, or not set at all, the call will be routed to an emergency centre. Note that this category element does not determine an emegency call, only the category of an emergency call. A call identified by the terminal as an emergency call will be sent with an EMERGENCY SETUP message rather than a SETUP message. Special characteristics of the EMERGENCY SETUP message include ignoring any called party number sent in that message. Note that none of the above mechanisms can guarantee that an emergency call is always identified as such at the phone, and therefore there is also a responsibility on the local serving network to do its own analysis as well. In the 3GPP SIP environment, that responsibility may well extend to other proxies, with the need to reroute back to the country of origin (all calls carry the mobile country code and mobile network code as well as potentially geographic information).