An IP Telephone, very broadly speaking, is a telephone designed to work with an IP PBX. The popularity of the SIP standard, however, means that the IP PBX of today has evolved, almost universally, into a SIP-based PBX.
This is good news for businesses and end users, because it means that PBX vendors can NO LONGER lock you into a custom phone brand or type. Phone manufacturers can produce SIP phones, and as long as the phones can support the SIP standard, then your investment is protected, and the phones can be used with virtually any SIP-based IP PBX.
Today’s modern SIP phones are also able to work without being constrained to a particular office or location. So we now can have on-premise phones and remote phones. Also, today’s mobile phones can run SIP software-based phones, allowing you to take your extension with you anywhere!
IP Phones are sometimes called VoIP telephones, SIP phones or softphones. These are just different names for a device/software client that is designed to support the transmission of voice over the internet, or what is better known as VoIP (or Voice over Internet Protocol) technology. In almost all cases, the supported protocol will be SIP.