Yealink SIP-T48G Administrator's Manual page 587

The call cannot be set up successfully.
User A
F1. INVITE B
F4. 100 Trying
F6. 486 Busy Here
F7. ACK
Step
INVITE—User A to Proxy
F1
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:
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

Appendix
User B
573