Yealink SIP-T48G Administrator's Manual page 885

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
INVITE—User A to Proxy
F1
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
Action
User B
Description
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:
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
865