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Voice technologies use dial peers to identify call origin and destination and to define the characteristics associated with a call leg. That way, calls can be exchanged smoothly between the packet network and the PSTN circuit-switched network. The router's voice-port hardware and software must be configured to transmit and receive the same type of signaling being used by the device with which they are interfacing. Voice ports on routers physically connect the router to telephony devices such as telephones, fax machines, PBXs, and PSTN central office (CO) switches. Cisco voice ports and dial peers are not specific features rather, they are the foundations on which all other voice features are built.

In the Cisco implementation of voice, voice ports define the physical interfaces and dial peers define the virtual interfaces to and from which a call is established. When configuring Cisco IOS voice features, it is important to understand the concepts of voice ports and dial peers. The call agent notifies the gateway to provide a dial tone. The gateway sends an event notification to the call agent, telling the agent that an off-hook condition has been detected. For example, when an MGCP gateway detects a telephone that has gone off hook, it does not know to automatically provide a dial tone. H.248, SCCP, and MGCP are examples of client/server protocols where the endpoints or gateways do not contain call control intelligence but send or receive event notifications to a server commonly referred to as a call agent.

SIP and H.323 are examples of peer-to-peer signaling protocols where the end devices or gateways contain the intelligence to initiate and terminate calls and interpret call control messages. Signaling protocols are classified as either peer-to-peer or client/server protocols. Some VoIP gateways are also capable of initiating SS7 signaling directly to the PSTN network.

VoIP presents several options for signaling, including H.323, Session Initiation Protocol (SIP), H.248, Media Gateway Control Protocol (MGCP), and Skinny Client Control Protocol (SCCP). SS7 uses out-of-band signaling, which is the exchange of call control information in a separate dedicated channel. The PSTN network uses Signaling System 7 (SS7) to transport control messages. Voice signaling requires the capability to provide supervisory, address, and alerting functionality between nodes. Is the capability to generate and exchange control information that will be used to establish, monitor, and release connections between two endpoints.