Yealink SIP-T2XP Administrator's Manual page 474

Administrator's Guide for SIP-T2xP and SIP-T19P IP Phones
Step
INVITE—User A to Proxy
F7
Server
INVITE—Proxy Server to User
F8
C
180 Ringing—User C to Proxy
F9
Server
180 Ringing—Proxy Server to
F10
User A
200OK—User C to Proxy
F11
Server
200OK—Proxy Server to User
F12
A
F13 ACK—User A to Proxy Server
F14 ACK—Proxy Server to User C
458
Action
User A sends a SIP INVITE request to the
proxy server. In the INVITE request, a
unique Call-ID is generated and the
Contact-URI field indicates that User A
requested the call.
The proxy server maps the SIP URI in the
To field to User C. The proxy server
sends the SIP INVITE request to User C.
User C sends a SIP 180 Ringing
response to the proxy server. The 180
Ringing response indicates that the user
is being alerted.
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.
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.
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 forwards the ACK
message to User C. The ACK confirms
that the proxy server has received the
200 OK response. The call session is
now active.
Description