Yealink SIP-T2XP Administrator's Manual page 451

Step
INVITE—Proxy Server to User
F2
B
100 Trying—User B to Proxy
F3
Server
100 Trying—Proxy Server to
F4
User A
486 Busy Here—User B to
F5
Proxy Server
486 Busy Here—Proxy Server
F6
to User A
F7 ACK—User A to Proxy Server
F8 ACK—Proxy Server to User B
The following figure illustrates the scenario of an unsuccessful call caused by the called
user's no answering. In this scenario, the two end users are User A and User B. User A
and User B are located at Yealink SIP IP phones.
Action
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 received by User B.
The proxy server forwards the SIP 100
Trying to User A to indicate that the
INVITE request has already been
received.
User B sends a SIP 486 Busy Here
response to the proxy server. The 486
Busy Here response is a client error
response indicating that User B is
successfully connected but User B is
busy on the IP phone and unable or
unwilling to take the call.
The proxy server forwards the 486 Busy
Here response to notify User A that User
B is busy.
User A sends a SIP ACK to the proxy
server. The SIP ACK message indicates
that User A has received the 486 Busy
Here message.
The proxy server forwards the SIP ACK
to User B to indicate that the 486 Busy
Here message has already been
received.
Description
specified.
Appendix
435