Administrator’s Guide for SIP-T2 Series/T19(P) E2/T4 Series/CP860 IP Phones
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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.
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.
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.
INVITE—Proxy Server to User
B
The proxy server forwards the mid-call
INVITE message to User B.
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 INVITE is
successfully processed.
200 OK—Proxy Server to User
A
The proxy server forwards the 200 OK
response to User A. The 200 OK
response notifies User B is successfully
placed on hold.
ACK—User A to Proxy Server
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.
ACK—Proxy Server to User B
The proxy server sends the ACK
message to User B. The ACK confirms
that the proxy server has received the
200 OK response.
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.
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