Yealink SIP-T54S Administrator's Manual page 1122

Administrator's Guide for SIP-T2 Series/T19(P) E2/T4 Series/T5 Series/CP860/CP920 IP Phones
Step
F20 200OK—Proxy Server to User A
F21 ACK— User A to Proxy Server
F22 ACK—Proxy Server to User C
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Action
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.
Description