Appendix B SIP Call Flows
Call Flow Scenarios for Failed Calls
| Step | Action | Description | ||
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1 | Call Setup is initiated between PBX A and Gateway 1. The | ||||
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| Call Setup includes the standard transactions that take | ||
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| place as User A attempts to call User B. | ||
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2 | Gateway 1 maps the SIP URL phone number to a | ||||
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| SIP IP phone | The | ||
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| of the SIP enabled entity to contact. Gateway 1 sends a SIP | ||
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| INVITE request to the address it receives as the dial peer | ||
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| which, in this scenario, is the Cisco SIP IP phone. | ||
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| In the INVITE request: | ||
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| • The IP address of the Cisco SIP IP phone is inserted in | ||
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| the | ||
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| • PBX A is identified as the call session initiator in the | ||
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| From field. | ||
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| • A unique numeric identifier is assigned to the call and | ||
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| is inserted in the | ||
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| • The transaction number within a single call leg is | ||
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| identified in the CSeq field. | ||
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| • The media capability User A is ready to receive is | ||
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| specified. | ||
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| • The port on which the Gateway is prepared to receive | ||
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| the RTP data is specified. | ||
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3 | Call | Gateway 1 sends a Call Proceeding message to PBX A to | |||
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| PBX A | acknowledge the Call Setup request. | ||
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4 | 100 | The Cisco SIP IP phone sends a SIP 100 Trying response | |||
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| to Gateway 1 | to Gateway 1. The 100 Trying response indicates that the | ||
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| INVITE request has been received by the Cisco SIP IP | ||
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| phone. | ||
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5 | 486 Busy | The Cisco SIP IP phone sends a SIP 486 Busy Here | |||
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| phone to Gateway 1 | response to Gateway 1. The 486 Busy Here response is a | ||
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| client error response that indicates that User B was | ||
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| successfully contacted but User B was not willing or was | ||
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| unable to take the call. | ||
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| Cisco SIP IP Phone 7960 Administrator Guide |
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