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Step

Action

Description

1.

Setup—PBX A to Gateway 1

Call setup is initiated between PBX A and Gateway 1. Setup includes the standard transactions that take place as User A attempts to call User B.

2.

INVITE—Gateway 1 to Cisco SIP IP phone

Gateway 1 maps the SIP URL phone number to a dial peer. The dial peer includes the IP address and the port number of the SIP-enabled entity to contact. Gateway 1 sends a SIP INVITE request to the address it receives as the dial peer, which, in this scenario, is the IP phone. In the INVITE request:

•The IP address of the phone is inserted in the Request-URI field.

•PBX A is identified as the call session initiator in the From field.

•A unique numeric identifier is assigned to the call and is inserted in the Call-ID field.

•The transaction number within a single call leg is identified in the CSeq field.

•The media capability that User A is ready to receive is specified.

•The port on which the gateway is prepared to receive the RTP data is specified.

3.

Call Proceeding—Gateway 1 to PBX A

Gateway 1 sends a Call Proceeding message to PBX A to acknowledge the Call Setup request.

4.

100 Trying—Cisco SIP IP phone to Gateway 1

The phone sends a SIP 100 Trying response to Gateway 1. The response indicates that the INVITE request has been received.

5.

180 Ringing—Cisco SIP IP phone to Gateway 1

The phone sends a SIP 180 Ringing response to Gateway 1. The response indicates that the user is being alerted.

6.

Alerting—Gateway 1 to PBX A

Gateway 1 sends an Alert message to User A. The message indicates that Gateway 1 has received a 180 Ringing response from the phone. User A hears the ringback tone that indicates that User B is being alerted.

7.

200 OK—Cisco SIP IP phone to Gateway 1

The phone sends a SIP 200 OK response to Gateway 1. The response notifies Gateway 1 that the connection has been made.

8.

Connect—Gateway 1 to PBX A

Gateway 1 sends a Connect message to the PBX A. The message notifies PBX A that the connection has been made.

9.

Connect ACK—PBX A to Gateway 1

PBX A acknowledges Gateway 1's Connect message.

10.

ACK—Gateway 1 to Cisco SIP IP phone

Gateway 1 sends a SIP ACK to the phone. The ACK confirms that Gateway 1 has received the 200 OK response. The call session is now active.

11.

BYE—Cisco SIP IP phone to Gateway 1

User B terminates the call session. The phone sends a SIP BYE request to Gateway 1. The request indicates that User B wants to release the call.

12.

Disconnect—Gateway 1 to PBX A

Gateway 1 sends a Disconnect message to PBX A.

13.

Release—PBX A to Gateway 1

PBX A sends a Release message to Gateway 1.

14.

200 OK—Gateway 1 to Cisco SIP IP phone

Gateway 1 sends a SIP 200 OK response to the phone. The response notifies the phone that Gateway 1 has received the BYE request.

15.

Release Complete—Gateway 1 to PBX A

Gateway 1 sends a Release Complete message to PBX A, and the call session terminates.