IP telephony is really just the business term for VoIP. It refers more to the device on your desk used to make a VoIP call than the VoIP call itself. TDM calls are made with conventional business phones, but unless equipped with a special adapter -- an ATA-type device -- these phones cannot make VoIP calls. An IP phone, on the other hand, is designed for VoIP, and is connected to the data network (or broadband router in small businesses) rather than the phone network.
When vendors talk about IP telephony, they are usually referring to the IP phones themselves, which businesses need to buy or lease when deploying VoIP. Implicit in this is the fact that the business is using them for VoIP. This is really mostly semantics, and in essence the two terms mean the same thing. The main difference would be that IP telephony is almost always used to refer to business telephony, whereas VoIP is used in both the consumer and business worlds.
This was first published in April 2008