Yealink SIP-T21 E2

Administrator’s Guide for SIP-T2 Series/T19(P) E2/T4 Series/T5 Series/CP860/CP920 IP Phones
1096
Step
Action
Description
F20
200OK—Proxy Server to User A
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.
F21
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.
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.