Administrator’s Guide for SIP-T2 Series/T19(P) E2/T4 Series/T5 Series/CP860/CP920 IP Phones
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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.
Unsuccessful Call Setup—Called User Does Not Answer
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.
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