Connecting a VoIP System to a Service Provider Network
A VoIP system that can place calls only to other devices on the IP network is only marginally useful; we still need to place calls out to the PSTN, and to do so we need to connect to a phone service provider, whether via traditional TDM links or ITSP connections. The device that acts as the interface to the PSTN is the voice gateway; it provides the physical connection and logical translation between two or more different network technologies.
Understanding Gateways, Voice Ports, and Dial Peers
The following sections establish some terms of reference.
In the Cisco Unified Communications architecture, a gateway is typically a voice-enabled router with an appropriate voice port installed ...