While the terms Voice over IP (VoIP) and IP Telephony are used interchangeably, they are actually not the same thing. In fact, VoIP is simply a subset of IP Telephony defining a particular application of the technology while IP Telephony is the general area of communications that involves digital phone systems using different IP protocols.
In this sense IP Telephony is the overall concept and VoIP is its implementation. If we think of an example from another field than electric cars would be the concept while electric-hybrid is one form of implementing this concept.
Beyond this mere definition IP Telephony is conceived as an overall business communication solution for a specific environment, including internal, external, intercom, and conferencing abilities. VoIP is more specifically a solution providing a cheaper alternative to traditional calling, hence it has been widely used in the consumer market.
Example: company X applies an overall IP Telephony to improve its productivity and reduce costs. This solution utilize VoIP technology and connects many workers. Worker Y of this company enjoys the features of this solution and subscribes to a consumer VoIP provider at his home. Now, he doesn’t own a complete IP Telephony solution, but simply makes and receives calls using VoIP instead of using traditional phone service.