Yealink SIP-T48G Administrator's Manual page 286

Microsoft Lync Edition Administrator's Guide for SIP-T2_Series and T4_Series IP Phones
The call flow scenario is as follows:
1. User A calls User B.
2. User B answers the call.
3. User A places User B on hold.
User A
F9. INVITE B (sendonly)
274
Proxy Server
F1. INVITE B
F4. 180 Ringing
F6. 200 OK
F7. ACK
2-way RTP channel established
F12. 200 OK
F13. ACK
No RTP packets being sent
F2. INVITE B
F3. 180 Ringing
F5. 200 OK
F8. ACK
F10. INVITE B (sendonly)
F11. 200 OK
F14. ACK
User B