User Manual

Yealink Skype for Business HD IP Phones Administrator Guide
418
Step
Action
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.
F2
INVITE—Proxy Server to User B
The proxy server maps the SIP URI in the To
field to User B. Proxy server forwards the
INVITE message to User B.
F3
180 Ringing—User B to Proxy
Server
User B sends a SIP 180 Ringing response to
the proxy server. The 180 Ringing response
indicates that the user is being alerted.
F4
180 Ringing—Proxy Server to
User A
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.
F5
200 OK—User B to Proxy Server
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.
F6
200 OK—Proxy Server to User A
The proxy server forwards the 200 OK
message to User A. The 200 OK response
notifies User A that the connection has
been made.
F7
ACK—User A to Proxy Server
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.
F8
ACK—Proxy Server to User B
The proxy server sends the SIP ACK to User
B. The ACK confirms that the proxy server
has received the 200 OK response. The call
session is now active.
F9
INVITE—User A to Proxy Server
User A sends a mid-call INVITE request to
the proxy server with new SDP session
parameters, which are used to place the call
on hold.