Yealink SIP-T2 SERIES Administrator's Manual page 1003

The call cannot be set up successfully.
User A
F1. INVITE B
F4. 100 Trying
F6. 486 Busy Here
F7. ACK
Step Action
INVITE—User A to Proxy
F1
Server
INVITE—Proxy Server to User
F2
B
F3
100 Trying—User B to Proxy
Proxy Server
F2. INVITE B
F3. 100 Trying
F5. 486 Busy Here
F8. ACK
Description
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 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.
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
Appendix
User B
981