The P-CSCF will send the response 380 Alternative Service to the originator of the
request, indicating that there will not be a dialog established, and no 2 xx response will
be sent. The request may be appended by the P-CSCF prior to forwarding to the destination
to indicate this is an emergency call.
There is still work underway to define how some government service will be supported,
such as the Government Emergency Telephone System (GETS) in the U.S. The
3GPP as well as the NS/EP IR working group are addressing the needs of government
for the prioritization of emergency calls (such as precedence in military applications)
and how these will be implemented in SIP and the IMS. This work will likely continue
over the next couple of years.
Modifying SIP Sessions in the IMS
Any established session can be modified while the session is in progress. For example,
during a conference call the originator may wish to add video into the call. This is
achieved by sending a new request to each participant. Any participant in an established
session can modify the session (dependent on applications, of course).
Note, however, that not all participants must receive this new request. If the originator
only wishes some of the participants to be part of the video session, the request is
sent only to those participants.
When the new request is sent, it will be populated with the same CALL ID as the
session in progress.
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