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Implementing Cisco Unified Communications Voice over IP and QoS (CVOICE) Foundation Learning Guide, Fourth Edition by Kevin Wallace

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Direct Inward Dialing Trunk

Typically, FXS ports connect to analog phones, but some carriers offer FXS trunks that support DID. The DID service is offered by telephone companies, and it enables callers to dial an extension directly on a PBX or a VoIP system (for example, Cisco Unified Communications Manager and Cisco IOS routers and gateways) without the assistance of an operator or automated call attendant. This service makes use of DID trunks, which forward only the last three to five digits of a phone number to the PBX, router, or gateway. For example, a company has phone extensions 555-1000 to 555-1999. A caller dials 555-1234, and the local CO forwards 234 to the PBX or VoIP system. The PBX or VoIP system then rings extension 234. This entire ...

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