Administrator’s Guide for SIP-T2 Series/T19(P) E2/T4 Series/T5 Series/CP860/CP920 IP Phones
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INVITE—User C to Proxy Server
User C sends a SIP INVITE message to the
proxy server. The INVITE request is an
invitation to User A to participate in a call
session.
In the INVITE request:
The IP address of User A is inserted
in the Request-URI field.
User C is identified as the call session
initiator in the From field.
A unique numeric identifier is
assigned to the call and is inserted in
the Call-ID field.
The transaction number within a
single call leg is identified in the
CSeq field.
The media capability User C is ready
to receive is specified.
The port on which User A is
prepared to receive the RTP data is
specified.
INVITE—Proxy Server to User A
The proxy server maps the SIP URI in the
To field to User A. The proxy server sends
the INVITE message to User A.
180 Ringing—User A to Proxy
Server
User A sends a SIP 180 Ringing response
to the proxy server. The 180 Ringing
response indicates that the user is being
alerted.
180 Ringing—Proxy Server to
User C
The proxy server forwards the 180 Ringing
response to User C. User C hears the ring-
back tone indicating that User A is being
alerted.
INVITE—User A to Proxy Server
User A sends a mid-call INVITE request to
the proxy server with new SDP session
parameters, which are used to place the
call on hold.
INVITE—Proxy Server to User B
The proxy server forwards the mid-call
INVITE message to User B.
200 OK—User B to Proxy Server
User B sends a 200 OK to the proxy server.
The 200 OK response indicates that the
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