Yealink T46G Administrator's Manual page 396

Yealink Skype for Business HD IP Phones Administrator Guide
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
382
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
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.
User B