Yealink SIP-T21 E2

Appendix
1085
Step
Action
Description
F1
INVITE—User A to Proxy Server
User A sends an INVITE message to a
proxy server. The INVITE request is an
invitation to User B to participate in a call
session.
In the INVITE request:
 The IP address of the User B is
inserted in the Request-URI field.
 User A is identified as the call
session initiator in the From field.
 A unique numeric identifier is
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
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.
F4
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 302
Move Temporarily message.
F5
302 Move Temporarily—Proxy
Server to User A
The proxy server forwards the 302 Moved
Temporarily message to User A.
F6
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
302 Move Temporarily message.
F7
INVITE—User A to Proxy Server
User A sends a SIP INVITE request to the
proxy server. In the INVITE request, a