Yealink SIP-T21 E2

Administrator’s Guide for SIP-T2 Series/T19(P) E2/T4 Series/T5 Series/CP860/CP920 IP Phones
1088
Step
Action
Description
assigned to the call and is inserted in
the Call-ID field.
 The transaction number within a
single call leg is identified in the
CSeq field.
 The media capability User A is ready
to receive is specified.
 The port on which User B is prepared
to receive the RTP data is specified.
F2
INVITE—Proxy Server to User B
The proxy server maps the SIP URI in the
To field to User B. The proxy server sends
the INVITE message to User B.
F3
180 Ringing—User B to Proxy
Server
User B sends a SIP 180 Ringing response
to the proxy server. The 180 Ringing
response indicates that the user is being
alerted.
F4
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 B is being
alerted.
F5
302 Move Temporarily—User B
to Proxy Server
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.
F6
ACK—Proxy Server to User B
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.
F7
302 Move Temporarily—Proxy
Server to User A
The proxy server forwards the 302 Moved
Temporarily message to User A.
F8
ACK—User A to Proxy Server
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.
F9
INVITE—User A to Proxy Server
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