Appendix
1013
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 was 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.
INVITE—User A to Proxy Server
User A sends a SIP INVITE request to the
proxy server. In the INVITE request, a
unique Call-ID is generated and the
Contact-URI field indicates that User A
requests the call.
INVITE—Proxy Server to User C
The proxy server maps the SIP URI in the To
field to User C. The proxy server sends the
INVITE request to User C.
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.
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.
200OK—User C to Proxy Server
User C sends a SIP 200 OK response to the
proxy server. The 200 OK response notifies
User A that the connection has been made.
200OK—Proxy Server to User A
The proxy server forwards the SIP 200 OK
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