A SIP IP Phone is actually a normal IP Telephone but it uses the SIP protocol for voice signalling communication. It can therefore allow a user to make and receive calls using the Voice over IP (VoIP) technology. The SIP telephone must be used with an IP Telephony system (IP PBX) that supports the SIP standard. Such IP Telephony systems are the Cisco CallManager, Avaya, Asterisk, etc.
There are two types of SIP Phones. A hardware SIP IP phone and a software SIP phone. The first type (hardware SIP phone) looks just like a regular telephone handset but it uses the Internet Protocol (IP) to communicate the voice conversation. The software SIP phones are used as software applications on a computer. They allow therefore any computer with a speaker and microphone to behave like a telephone. You also need a broadband internet connection and a subscription service with a VoIP provider or with a SIP proxy server.