B-48
Cisco Unified IP Phone 7960G and 7940G Administration Guide for Release 8.0 (SIP)
OL-7890-01
Appendix B SIP Call Flows
Call Flow Scenarios for Failed Calls
Figure B-18 Called Number is Busy
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 A attempts to call 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 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.
486 Busy Here—Cisco SIP IP phone to
Gateway 1
The phone sends a SIP 486 Busy Here response to Gateway 1. The
response is a client error response that indicates that B was successfully
contacted but that B was not willing or was unable to take the call.
6.
Disconnect (Busy)—Gateway 1 to
PBX A
Gateway 1 sends a Disconnect message to PBX A.
7.
Release—PBX A to Gateway 1
PBX A sends a Release message to Gateway 1.
IP
3. Call Proceeding
6. Disconnect (Busy)
1. Setup
7. Release
PBX A
User A
GW1
IP Network
4. 100 Trying
5. 486 Busy Here
8. ACK
2. INVITE
9. Release Complete
41725
SIP IP Phone
User B