Appendix B SIP Call Flows

Call Flow Scenarios for Successful Calls

Step

Action

 

Description

 

 

 

 

6

Alerting—Gateway 1 to PBX A

 

Gateway 1 sends an Alert message to User A. The Alert

 

 

 

 

 

message indicates that Gateway 1 has received a 180

 

 

 

 

 

Ringing response from the Cisco SIP IP phone. User A

 

 

 

 

 

hears the ringback tone that indicates that User B is being

 

 

 

 

 

alerted.

 

 

 

 

7

200 OK—Cisco SIP IP phone to

 

The Cisco SIP IP phone sends a SIP 200 OK response to

 

Gateway 1

 

Gateway 1. The 200 OK response notifies Gateway 1 that

 

 

 

 

 

the connection has been made.

 

 

 

 

8

Connect—Gateway 1 to PBX A

 

Gateway 1 sends a Connect message to PBX A. The

 

 

 

 

 

Connect message notifies PBX A that the connection has

 

 

 

 

 

been made.

 

 

 

 

9

ACK—Gateway 1 to Cisco SIP

 

Gateway 1 sends a SIP ACK to the Cisco SIP IP phone. The

 

IP phone

 

ACK confirms that Gateway 1 has received the 200 OK

 

 

 

 

 

response. The call session is now active.

 

 

 

 

10

Connect ACK—PBX A to

 

PBX A acknowledges Gateway 1’s Connect message.

 

Gateway 1

 

 

 

 

 

 

 

11

BYE—Cisco SIP IP phone to

 

User B transfers User A’s call to User C and then hangs up.

 

Gateway 1

 

The Cisco SIP IP phone sends a SIP BYE request to

 

 

 

 

 

Gateway 1. The SIP BYE request includes the Also header.

 

 

 

 

 

In this scenario, the Also header indicates that User C

 

 

 

 

 

needs to be brought into the call while User B hangs up.

 

 

 

 

 

This header distinguishes the call transfer BYE request

 

 

 

 

 

from a normal disconnect BYE request.

 

 

 

 

 

The Request-By header could be included in the BYE

 

 

 

 

 

request, however, Cisco’s implementation does not require

 

 

 

 

 

the header to complete the transfer. If the Requested-By

 

 

 

 

 

header is included, the INVITE sent to the transferred-to

 

 

 

 

 

party will include the Requested-By header. If the

 

 

 

 

 

Requested-By header is not included, the INVITE sent to

 

 

 

 

 

the transferred-to party will not include the Requested-By

 

 

 

 

 

header.

 

 

 

 

12

200 OK—Gateway 1 to Cisco

 

Gateway 1 sends a SIP 200 OK message to the Cisco SIP

 

SIP IP phone

 

IP phone. The 200 OK response notifies the Cisco SIP IP

 

 

 

 

 

phone that the BYE request has been received. The call

 

 

 

 

 

session between User A and User B is now terminated.

 

 

 

 

 

 

 

 

 

Cisco SIP IP Phone 7960 Administrator Guide

 

 

 

B-14

 

 

 

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