Yealink SIP-T2 SERIES Administrator's Manual page 1077

Call is established between User A and User C.
User A
F1. INVITE B
F4. 180 Ringing
F7. 302 Move Temporarily
F8. ACK
F9. INVITE C
F12. 180 Ringing
F14. 200 OK
F15. ACK
Step Action
F1 INVITE—User A to Proxy Server
Proxy Server
F2. INVITE B
F3. 180 Ringing
F5. 302 Move Temporarily
F6. ACK
F10. INVITE C
F11. 180 Ringing
F13. 200 OK
F16. ACK
2-way RTP channel established
User A sends the 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:




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User B
Description
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.
Appendix
User C
1053