Yealink SIP-T2 Series - Page 1051

Appendix
1027
Step
Action
Description
The ACK confirms that User A has received
the 200 OK response. The call session is
now active.
F22
ACK—Proxy Server to User C
The proxy server sends the ACK message to
User C. The ACK confirms that the proxy
server has received the 200 OK response.