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An IP phone also known as a VOIP/SIP phone, is a software/instrument specially made to be used for VoIP. IP phones make utilize VoIP (Voice Over Internet Protocol) transmission technologies to make and receive phone calls over an IP network which are not necessarily made over a common PSTN system. While IP service providers do employ normal PSTN instruments as VoIP phones by attaching an Analog Telephony Adapter (ATA), there are specially made VoIP phones by brands like Cisco, Linksys, Philips, etc. who have ventured into this market.
IP phones are not all about phones that can be physically seen and touched (known as "hardphones"), but also about software based "softphones" which operate on the path of Internet via protocols such as Session Initiation Protocol, Skinny Client Control Protocol or protocols that are specific to the service provider. These softphones are internet based but their user interface is similar to that of a dial up telephone with a numeric keypad, buttons for receiving/ending a call, etc. Many IP phones use protocols provided specifically by their service providers (e.g. Skype), but most of them support Session Initiation Protocol (SIP) and therefore are also known as SIP phones.
