IP phone

A VoIP phone or IP phone uses Voice over IP technologies for placing and transmitting telephone calls over an IP network, such as the Internet, instead of the traditional public switched telephone network (PSTN).

Digital IP-based telephone service uses control protocols such as the (SIP), (SCCP) or various other .

VoIP phones can be simple software-based softphones or purpose-built hardware devices that appear much like an ordinary telephone or a cordless phone. Traditional PSTN phones are used as VoIP phones with analog telephone adapters (ATA).

Two combined signal-and-power wired cable interfaces are in common use to communicate between computer networks (or computers) and physically separate VoIP phones: USB and Power over Ethernet. The latter is preferred in industry such as large scale call centers or PBX replacement applications, because PoE has the following advantages over USB:

For these reasons, USB and softphone PC applications are considered transitional by many industrial users and makers of larger telephone switches, used only to build markets for VoIP that will eventually shift over to the more robust PoE technology shipped by Cisco, Siemens, Alcatel and other large switch makers.

A VoIP phone or application may have many features an analog phone doesn't support, such as e-mail-like IDs for contacts that may be easier to remember than names or phone numbers, or easy sharing of contact lists among multiple accounts. Generally the features of VoIP phones follow those of Skype, Google Voice and other PC-based phone services, which have richer feature sets but (because they rely on mainstream operating systems' IP support) latency-related audio problems.

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