Yealink SIP-T2 Series

Appendix
997
Step
Action
Description
indicates that the INVITE request has been
received by User B.
F4
100 Trying—Proxy Server to User
A
The proxy server forwards the SIP 100
Trying to User A to indicate that the INVITE
request has already been received.
F5
486 Busy Here—User B to Proxy
Server
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.
F6
486 Busy Here—Proxy Server to
User A
The proxy server forwards the 486 Busy
Here response to notify User A that User B
is busy.
F7
ACK—User A to Proxy Server
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.
F8
ACK—Proxy Server to User B
The proxy server forwards the SIP ACK to
User B to indicate that the 486 Busy Here
message has already been received.
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.
The call flow scenario is as follows:
1. User A calls User B.
2. User B does not answer the call.
3. User A hangs up.