Yealink SIP-T2 SERIES Administrator's Manual page 1023

4. User C answers the call.
Call is established between User A and User C.
User A
F1. INVITE B
F5. 302 Move Temporarily
F6. ACK
F7. INVITE C
F10. 180 Ringing
F12. 200 OK
F13. ACK
Step
INVITE—User A to Proxy
F1
Server
Proxy Server
F2. INVITE B
F3. 302 Move Temporarily
F4. ACK
F8. INVITE C
F9. 180 Ringing
F11. 200 OK
F14. ACK
2-way RTP channel established
Action
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:
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


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User B
Description
The IP address of the 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
Appendix
User C
1001