INVITE—User A to Proxy
Server
User A sends a SIP INVITE request to the
proxy server. In the INVITE request, a
unique Call-ID is generated and the
Contact-URI field indicates that User A
requested the call.
INVITE—Proxy Server to User
C
The proxy server maps the SIP URI in the
To field to User C. The proxy server
sends the SIP INVITE request to User C.
180 Ringing—User C to Proxy
Server
User C sends a SIP 180 Ringing
response to the proxy server. The 180
Ringing response indicates that the user
is being alerted.
180 Ringing—Proxy Server to
User A
The proxy server forwards the 180
Ringing response to User A. User A
hears the ring-back tone indicating that
User C is being alerted.
200OK—User C to Proxy
Server
User C sends a SIP 200 OK response to
the proxy server. The 200 OK response
notifies User A that the connection has
been made.
200OK—Proxy Server to User
A
The proxy server forwards the SIP 200
OK response to User A. The 200 OK
response notifies User A that the
connection has been made.
ACK—User A to Proxy Server
User A sends a SIP ACK to the proxy
server. The ACK confirms that User A
has received the 200 OK response. The
call session is now active.
ACK—Proxy Server to User C
The proxy server forwards the ACK
message to User C. The ACK confirms
that the proxy server has received the
200 OK response. The call session is
now active.
The following figure illustrates successful call forwarding between Yealink SIP IP phones
in which User B has enabled busy call forward. The incoming call is forwarded to User C
when User B is busy. In this call flow scenario, the end users are User A, User B, and User
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