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 | |
| 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 |
| PBX A | acknowledge the Call Setup request. |
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4 | 100 | The Cisco SIP IP phone sends a SIP 100 Trying response |
| 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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| Cisco SIP IP Phone 7960 Administrator Guide |
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