Telephoney over IP can referred to a "service" having your telephone service at your home is just IP enabled where what traditional telephony services can provide will be just transparent to IP Telephony.
On the other hand, VoIP can be referred to as one of your applications within your local network. You may have VoIP calls with anyone within your lcoal network but you may not be able to have a telephoney service, meaning can't call your neighbor accross the street, unless you have a gateway to PSTN.
However, from a technology standpoint, IP Telephony or VoIP are both IP enabled vocie packet.
