Yealink SIP-T2 SERIES Administrator's Manual page 1035

Step Action
200OK—Proxy Server to User
F20
A
F21 ACK— User A to Proxy Server
F22 ACK—Proxy Server to User C
Description
been made.
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.
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.
The proxy server sends the ACK
message to User C. The ACK confirms
that the proxy server has received the
200 OK response.
Appendix
1013