200 OK—User B to Proxy Server
User B sends a SIP 200 OK response to the
proxy server. The 200 OK response
notifies User A that the INVITE is
successfully processed.
200 OK—Proxy Server to User A
The proxy server forwards the 200 OK
response to User A. The 200 OK response
notifies User B is successfully placed on
hold.
ACK—User A to Proxy Server
User A sends an ACK message to the
proxy server. The ACK confirms that User
A has received the 200 OK response. The
call session is now temporarily inactive.
No RTP packets are being sent.
ACK—Proxy Server to User B
The proxy server sends the ACK message
to User B. The ACK confirms that the
proxy server has received the 200 OK
response.
Successful Call Setup and Call Waiting
The following figure illustrates a successful call between Yealink SIP IP phones in which two
parties are in a call, one of the participants receives and answers an incoming call from a third
party. 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 C calls User B.
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