There is none.
Actually VoIP means ‘voice over IP’
This is the technical way to encode and transmit (forward) your voice.The standard Technic isn’t realeased yet but when it will, you’ll be able to give a phone call via internet from your single telephone(+special card) or from your PC (H323 station).
Internet Telephony is often a ‘solution pack’ (software) which allows you to call somebody (with or without video interface/ and with or without chat box).
You’ll ring somebody via your PC.
I don’t know weither or not if you can read french or if you can translate the web page but let’s check out: http://www.guill.net