VoIP (or Voice over IP), is quickly becoming the new standard in business telephone. All Nortel Norstar phone systems prior to the BCM do not directly support VoIP. However, VoIP can still be used with a Norstar system via the use of ATA (Analog Terminal Adapters).
The Nortel BCM product actually is setup with VoIP support directly.
If you're considering adding VoIP to your Norstar and you're in the United States, I have a handful of favorit VoIP providers I work with. The right one will depend on how many lines you need, the type of Internet connection you have, and your calling patterns. Contact me for a quick discussion and I'll connect you with the best provider based on a your situation.