Yealink SIP-T2 SERIES Administrator's Manual page 1054

Administrator's Guide for SIP-T2 Series/T19(P) E2/T4 Series/CP860 IP Phones
The call cannot be set up successfully.
User A
Step
F1 INVITE—User A to Proxy Server
F2 INVITE—Proxy Server to User B
180 Ringing—User B to Proxy
F3
Server
1030
Proxy Server
F1. INVITE B
F4. 180 Ringing
F5. CANCEL
F8. 200 OK
Action
F2. INVITE B
F3. 180 Ringing
F6. CANCEL
F7. 200 OK
Description
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.
The proxy server maps the SIP URI in the To
field to User B. Proxy server forwards the
INVITE message to User B.
User B sends a SIP 180 Ringing response to
the proxy server. The 180 Ringing response
indicates that the user is being alerted.
User B