Specifications

Appendix
421
Step
Action
Description
F18
INVITE—Proxy Server to User C
The proxy server maps the SIP URI in the
To field to User C.
F19
180 Ringing—User C to Proxy
Server
User C sends a SIP 180 Ringing response
to the proxy server. The 180 Ringing
response indicates that the user is being
alerted.
F20
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
C is being alerted
F21
200OK—User C to Proxy Server
User C sends a SIP 200 OK response to
the proxy server. The 200 OK response
notifies the proxy server that the
connection has been made.
F22
200OK—Proxy Server to User A
The proxy server forwards the SIP 200 OK
response to User A.
F23
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.
F24
ACK—Proxy Server to User C
The proxy server forwards the ACK
message to User C. The ACK confirms
that User A has received the 200 OK
response. The call session is now active.
Call Transfer with Consultation
The following figure illustrates a successful call between Yealink SIP IP phones in which
two parties are in a call and then one of the parties transfers the call to the third party
with consultation. This is called attended transfer. In this call flow scenario, the end users
are User A, User B, and User C. They are all using Yealink SIP IP phones, which are
connected via an IP network.
The call flow scenario is as follows:
1. User A calls User B.
2. User B answers the call.
3. User A calls User C.
4. User C answers the call.
5. User A transfers the call to User C.