INVITE—Proxy Server to User
B
The proxy server maps the SIP URI in the
To field to User B. Proxy server forwards
the INVITE message to User B.
100 Trying—User B to Proxy
Server
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.
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.
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.
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.
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.
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.
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