Yealink SIP-T48G Administrator's Manual page 284

Microsoft Lync Edition Administrator's Guide for SIP-T2_Series and T4_Series IP Phones
The call flow scenario is as follows:
1. User A calls User B.
2. User B does not answer the call.
3. User A hangs up.
The call cannot be set up successfully.
User A
F1. INVITE B
F4. 180 Ringing
F5. CANCEL
F8. 200 OK
Step
INVITE—User A to Proxy
F1
Server
272
Proxy Server
Action
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:





F2. INVITE B
F3. 180 Ringing
F6. CANCEL
F7. 200 OK
Description
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
User B