INVITE—Proxy Server to User C
The proxy server forwards 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.
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 sends the ACK message
to User C.
No Answer Call Forward
The following figure illustrates successful call forwarding between Yealink SIP IP phones in
which User B has enabled no answer call forward. The incoming call is forwarded to User C
when User B does not answer the incoming call after a period of time. In this call flow scenario,
the end users are User A, User B, and User C. They are all using Yealink SIP IP phones, which are
connected via an IP network.
The call flow scenario is as follows:
1. User B enables no answer call forward, and the destination number is User C.
2. User A calls User B.
3. User B does not answer the incoming call.
4. User B forwards the incoming call to User C.
5. User C answers the call.
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