Yealink SIP-T2XP Administrator's Manual page 559

Step
INVITE—Proxy Server to User
F10
B
200 OK—User B to Proxy
F11
Server
200 OK—Proxy Server to User
F12
A
F13 ACK—User A to Proxy Server
F14 ACK—Proxy Server to User B
The following figure illustrates a successful call between Yealink SIP IP phones in which
two parties are in a call, one of the participants receives and answers an incoming call
from a third party. 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 C calls User B.
Action
place the call on hold.
The proxy server forwards the mid-call
INVITE message to User B.
User B sends a SIP 200 OK response to
the proxy server. The 200 OK response
notifies User A that the INVITE is
successfully processed.
The proxy server forwards the 200 OK
response to User A. The 200 OK
response notifies User B is successfully
placed on hold.
User A sends an ACK message to the
proxy server. The ACK confirms that
User A has received the 200 OK
response. The call session is now
temporarily inactive. No RTP packets
are being sent.
The proxy server sends the ACK
message to User B. The ACK confirms
that the proxy server has received the
200 OK response.
Appendix
Description
541