Yealink SIP-T48G Administrator's Manual page 886

Administrator's Guide for SIP-T2 Series/T19(P) E2/T4 Series/CP860 IP Phones
Step
INVITE—Proxy Server to User
F2
B
180 Ringing—User B to Proxy
F3
Server
180 Ringing—Proxy Server to
F4
User A
302 Move Temporarily—User
F5
B to Proxy Server
F6 ACK—Proxy Server to User B
302 Move Temporarily—Proxy
F7
Server to User A
F8 ACK—User A to Proxy Server
INVITE—User A to Proxy
F9
Server
INVITE—Proxy Server to User
F10
C
180 Ringing—User C to Proxy
F11
Server
866
Action

The proxy server maps the SIP URI in the
To field to User B. The proxy server
sends the INVITE message to User B.
User B 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 B is being alerted.
User B sends a SIP 302 Moved
Temporarily message to the proxy
server. The message indicates that User
B is not available at SIP phone B. User B
rewrites the contact-URI.
The proxy server sends a SIP ACK to
User B, the ACK message notifies User B
that the proxy server has received the
ACK message.
The proxy server forwards the 302
Moved Temporarily message to User A.
User A sends a SIP ACK to the proxy
server. The ACK message notifies the
proxy server that User A has received
the ACK message.
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
requests the call.
The proxy server forwards the SIP
INVITE request to User C.
User C sends a SIP 180 Ringing
response to the proxy server. The 180
Description
The port on which User B is
prepared to receive the RTP data is
specified.