Yealink SIP-T48G Administrator's Manual page 862

Administrator's Guide for SIP-T2 Series/T19(P) E2/T4 Series/CP860 IP Phones
The call cannot be set up successfully.
User A
F1. INVITE B
F4. 100 Trying
F6. 486 Busy Here
F7. ACK
Step
INVITE—User A to Proxy
F1
Server
INVITE—Proxy Server to User
F2
B
F3
100 Trying—User B to Proxy
842
Proxy Server
Action
User A sends the 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 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 100 Trying response
F2. INVITE B
F3. 100 Trying
F5. 486 Busy Here
F8. ACK
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
ready to receive is specified.
The port on which User B is
prepared to receive the RTP data is
specified.
User B