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 generally intended as an overall business communication solution. It supports internal and PBX calling, external calling, faxing, text messaging, intercom, door phone, and conferencing. VoIP is more specifically a cheaper alternative to traditional calling (although it can be used for more purposes) and has been widely adopted in the consumer market.
Example: A Company applies an IP Telephony solution to improve productivity and reduce costs. This solution utilize VoIP technology and connects workers with a cloud hosted PBX. A worker of this company 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 a traditional phone service.