IP telephone systems are, in their most basic functional form, almost identical to conventional telephone systems except that the delivery of the telephone extension is done via a computer network, or an IP network. Better described as an IP-PBX, IP telephone systems interface to normal public switched telephone network lines for the external carriage of a telephone call.
Voice Over IP (“VoIP”) on the other hand is a term used to describe various telephony transmission technologies that interface to an IP network (typically the Internet) for the external carriage of a telephone call.
Though the two systems (or at least the functions of the two systems) may be found in one product offering, the fact remains that they are fundamentally different technological systems that should be considered separately during any evaluation.
The following table is intended to help clarify the difference between IP telephony and VoIP: