Many people use the terms voice over internet protocol (VoIP) and IP telephony (IPT) interchangeably, equating the two. However, VoIP is only a subset of IP telephony. The difference between the two terms is so subtle that they generally mean the same thing.

It may sound confusing, but "telephony" refers to telephones, and Internet Protocol telephony deals with the digital side of telecommunications. It does this with the Internet Protocol known as voice over IP (VoIP).

What this literally means is that you are transferring voice signals over the Internet. The protocol defines how voice travels over a network, similar to how the hypertext transfer protocol (HTTP) defines how data should be understood, transmitted, formatted, and displayed by web servers and browsers.

To see it in a broader perspective, think of IP telephony as the overall concept and VoIP as a way of transmitting voice to implement this concept. For example, an IP telephony system can be an IP-PBX (private branch exchange), which has VoIP and its standards (SIP, H.323, etc.), along with many other things (e.g. CRM), aimed at better productivity.