Appendix B SIP Call Flows
Call Flow Scenarios for Successful Calls
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Cisco SIP IP Phone 7960 Administrator Guide
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200 OK—Cisco SIP IP phone B
to Cisco SIP IP phone A
Cisco SIP IP phone B sends a SIP 200 OK response to
Cisco SIP IP phone A. The 200 OK response notifies Cisco
SIP IP phone A that the connection has been made.
If Cisco SIP IP phone B supports the media capability
advertised in the INVITE message sent by Cisco SIP IP
phone A, it advertises the intersection of its own and Cisco
SIP IP phone A’s media capability in the 200 OK response.
If Cisco SIP IP phone B does not support the media
capability advertised by Cisco SIP IP phone A, it sends
back a 400 Bad Request response with a 304 Warning
header field.
4
ACK—Cisco SIP IP phone A to
Cisco SIP IP phone B
Cisco SIP IP phone A sends a SIP ACK to Cisco SIP IP
phone B. The ACK confirms that Cisco SIP IP phone A has
received the 200 OK response from Cisco SIP IP phone B.
The ACK might contain a message body with the final
session description to be used by Cisco SIP IP phone B. If
the message body of the ACK is empty, Cisco SIP IP
phone B uses the session description in the INVITE
request.
A two-way RTP channel is established between Cisco SIP IP phone A and Cisco SIP IP phone B.
User B then selects the option to transfer the call to User C.
5
BYE—Cisco SIP IP phone B to
Cisco SIP IP phone A
The call continues and then User B hangs up. Cisco SIP IP
phone B sends a SIP BYE request to Cisco SIP IP phone A.
The SIP BYE request includes the Also header. The Also
header indicates that User C needs to be brought into the
call while User B hangs up. The header distinguishes the
call transfer BYE request from a normal disconnect BYE
disconnect BYE request.
6
200 OK—Cisco SIP IP phone A
to Cisco SIP IP phone B
Cisco SIP IP phone A sends a SIP 200 OK message to
Cisco SIP IP phone B. The 200 OK response notifies Cisco
SIP IP phone B that the BYE request has been received.
The call session between User A and User B is now
terminated.
The RTP channel between Cisco SIP IP phone A and Cisco SIP IP phone B is torn down.
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