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Microsoft Lync Edition Administrator’s Guide for SIP-T2_Series and T4_Series IP Phones
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Step Action Description
F18
180 Ringing—Proxy Server to
User A
The proxy server forwards the 180
Ringing response to User A. User A
hears the ring-back tone indicating that
User C is being alerted.
F19
200OK—User C to Proxy
Server
User C sends a SIP 200 OK response to
the proxy server. The 200 OK response
notifies User A that the connection has
been made.
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.