Yealink SIP-T2 SERIES Administrator's Manual page 1007

3. User A places User B on hold.
User A
F1. INVITE B
F4. 180 Ringing
F6. 200 OK
F7. ACK
F9. INVITE B (sendonly)
F12. 200 OK
F13. ACK
Step
INVITE—User A to Proxy
F1
Server
Proxy Server
2-way RTP channel established
No RTP packets being sent
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:




F2. INVITE B
F3. 180 Ringing
F5. 200 OK
F8. ACK
F10. INVITE B (sendonly)
F11. 200 OK
F14. ACK
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
Appendix
User B
985