Yealink Yealink SIP-T42G, Administrator'S Manual

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Administrator’s Guide for SIP-T2 Series/T19(P) E2/T4 Series/CP860 IP Phones
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4. User C answers the call.
Call is established between User A and User C.
User A Proxy Server User B User C
F1. INVITE B
F2. INVITE B
F3. 302 Move Temporarily
F6. ACK
F7. INVITE C
F8. INVITE C
F9. 180 Ringing
F10. 180 Ringing
F11. 200 OK
F12. 200 OK
F13. ACK
F14. ACK
F4. ACK
2-way RTP channel established
F5. 302 Move Temporarily
Step Action Description
F1
INVITE—User A to Proxy
Server
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:

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.