Yealink T46G Administrator's Manual page 394

Yealink Skype for Business HD IP Phones Administrator Guide
Step
F2 INVITE—Proxy Server to User B
100 Trying—User B to Proxy
F3
Server
100 Trying—Proxy Server to User
F4
A
180 Ringing—User B to Proxy
F5
Server
180 Ringing—Proxy Server to
F6
User A
F7 200 OK— User B to Proxy Server
F8 200OK—Proxy Server to User A
F9 ACK—User A to Proxy Server
380
Action
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The proxy server maps the SIP URI in the To
field to User B. The proxy server sends the
INVITE message to User B.
User B sends a SIP 100 Trying response to
the proxy server. The 100 Trying response
indicates that the INVITE request has been
received by User B.
The proxy server forwards the SIP 100
Trying to User A to indicate that the INVITE
request has been received by User B.
User B sends a SIP 180 Ringing response to
the proxy server. The 180 Ringing response
indicates that the User B is being alerted.
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.
User B sends a SIP 200 OK response to the
proxy server. The 200 OK response notifies
User A that the connection has been made.
The proxy server forwards the 200 OK
message to User A. The 200 OK response
notifies User A that the connection has
been made.
User A sends a SIP ACK to the proxy server.
The ACK confirms that User A has received
the 200 OK response. The call session is
now active.
Description
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.