Yealink W60P Administrator's Manual page 514

Administrator's Guide for W60P DECT IP Phones
The call cannot be set up successfully.
User A
Step
F1 INVITE—User A to Proxy Server
F2 INVITE—Proxy Server to User B
180 Ringing—User B to Proxy
F3
Server
494
Proxy Server
F1. INVITE B
F4. 180 Ringing
F5. CANCEL
F8. 200 OK
Action
F2. INVITE B
F3. 180 Ringing
F6. CANCEL
F7. 200 OK
Description
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 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.
The proxy server maps the SIP URI in the
To field to User B. Proxy server forwards
the INVITE message to User B.
User B sends a SIP 180 Ringing response
to the proxy server. The 180 Ringing
User B