Yealink SIP-T19 E2 T4 Series, Administrator'S Manual

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Administrator’s Guide for SIP-T2 Series/T19(P) E2/T4 Series/T5 Series IP Phones
926
The call cannot be set up successfully.
User A Proxy Server User B
F1. INVITE B
F2. INVITE B
F3. 180 Ringing
F4. 180 Ringing
F8. 200 OK
F7. 200 OK
F5. CANCEL
F6. CANCEL
Step Action Description
F1 INVITE—User A to Proxy Server
User A sends an INVITE message to a proxy
server. The INVITE request is an invitation to
User B to participate in a call session.
In the INVITE request:

The IP address of User B is inserted in
the Request-URI field.

User A 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 A is ready
to receive is specified.

The port on which User B is prepared
to receive the RTP data is specified.
F2 INVITE—Proxy Server to User B
The proxy server maps the SIP URI in the To
field to User B. Proxy server forwards the
INVITE message to User B.
F3
180 Ringing—User B to Proxy
Server
User B sends a SIP 180 Ringing response to
the proxy server. The 180 Ringing response
indicates that the user is being alerted.