- Baylink says: This is my best understanding and phrasing of the difference; I expect this definition to take fire and revision. :-)
Internet telephony differs slightly from VoIP (Voice over IP) in that it specifically involves interconnections with the traditional PSTN, either in that a call is made between a PSTN phone and an Internet phone/program, or that the Internet is used to haul a call that's going from one spot on the PSTN to another over the Internet.
VoIP need not transit the public Internet, but Internet telephony both does, and interacts with the PSTN.
One problem which seems to occur in the interaction of VoIP phones and networking equipment relates to the packet per second rate of the equipment: some networking gear apparently doesn't deal with with fast flows of tiny packets, according to an April 2005 NANOG thread. One solution, if the knob is available to you, is to enlarge the packet size for a given codec; this reduces the packet count.