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Yealink SIP-T54S Administrator's Manual page 1091

Sip-t2 series; sip-t4 series; sip-t5 series
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The call cannot be set up successfully.
User A
F1. INVITE B
F4. 100 Trying
F6. 486 Busy Here
F7. ACK
Step
F1
INVITE—User A to Proxy Server
F2
INVITE—Proxy Server to User B
100 Trying—User B to Proxy
F3
Server
Proxy Server
Action
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 proxy server maps the SIP URI in the
To field to User B. Proxy server forwards
the INVITE message to User B.
User B sends a SIP 100 Trying response to
the proxy server. The 100 Trying response
indicates that the INVITE request has been
F2. INVITE B
F3. 100 Trying
F5. 486 Busy Here
F8. 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
CSeq field.
The media capability User A is ready
to receive is specified.
The port on which User B is prepared
to receive the RTP data is specified.
Appendix
User B
1065

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