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 the proxy server 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 forwards the ACK
message to User C. The ACK confirms that
User A has received the 200 OK response.
The call session is now active.
Call Transfer with Consultation
The following figure illustrates a successful call between Yealink SIP IP phones in which two
parties are in a call and then one of the parties transfers the call to the third party with
consultation. This is called attended transfer. 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 A calls User B.
2. User B answers the call.
3. User A calls User C.
4. User C answers the call.
5. User A transfers the call to User C.
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