To increase the likelihood that diverse user equipment can successfully
communicate with the PSAP, it is recommended that call taker equipment
has at least the following capabilites:
RFC 3261, with
UDP, TCP and TLS (sips) support.
RTP and RTCP according to RFC 3550 and RFC
3551. SRTP according to RFC 3711.
G.711, GSM 06.10, DTMF support using RFC 2833, with forward error correction (RFC 2733).
using RTP according to RFC 2793bis.
Support H.261, H.263 and H.264 in QCIF, CIF and 4CIF
sizes.
RFC
3428
A user agent placing an emergency call SHOULD use the "sips" URI
schema for all such calls, forcing these calls to use TLS as secure hop-by-hop
transport. If a call cannot be established using TLS transport, the user
agent SHOULD attempt a call using the "sip" URI.
If a user agent receives a redirect (3xx) response for an emergency
call, it MUST include the location information contained in that
response in the outgoing call. This differs from regular behavior for redirects,
where the message body is not copied into the new call.
User agents MUST support blind transfer using REFER.
A user agent MUST check the Contact URI in redirect responses to see
if it is an emergency call, as described in . If so, the behavior in the previous paragraph
applies.
End systems that allow human users to initiate an emergency call with
a single button press or other similar stimulus SHOULD require callers
to confirm their call.
UAs SHOULD place a "Priority" header with value "emergency" in all
emergency calls, but its presence cannot be relied upon to identify an
emergency call.