How does a VoIP phone work?

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Some VoIP phones require A/C adapters for power, while others use Power over Ethernet (PoE). PoE uses an Ethernet cable instead of an A/C adapter and removes the need for separate power and data cables.

Several networking components are required to make VoIP phones work. Phones are assigned IP addresses through the Dynamic Host Configuration Protocol (DHCP), which automatically configures the network and the VoIP parameters. A domain name system (DNS) tracks the IP addresses to enable devices, such as IP phones, to connect to each other.

VoIP phones require a number of protocols to facilitate the delivery of voice communications over the internet. H.323 is the most commonly used VoIP protocol that supports audio, video and data communications across IP networks. It provides several VoIP functions, including bandwidth management and call control.

Session initiation protocol (SIP) is a signaling protocol that sets up VoIP connections and is used as an alternative to H.323. Real-Time Transport Protocol (RTP) is used to send and receive multimedia information between two devices. VoIP services use RTP and SIP or H.323 to stream multimedia content. The Simple Traversal of UDP through NAT (STUN) protocol is used on some SIP-based VoIP phones to enable communications behind network firewalls, which can sometimes block SIP and RTP packets.

Some providers use their own proprietary protocols for VoIP phones. For example, the Skinny Client Control Protocol (SCCP) is a proprietary Cisco standard for communicating with H.323 VoIP systems.