Cisco NAC & IP Phones

I've been told that when you implement Cisco NAC and are using non-Cisco IP phones, the device will authenticate the phone but not the pc plugged into it. Is this true and if so, is there a fix or a workaround? Nortel has been using this as one of their key points to discourage us from buying all Cisco routers and implementing NAC.

We are using Nortel phone systems right now but they aren't IP based. However, it seems it would be cheaper to upgrade those instead of replacing them with Cisco units.

If we buy Nortel IP phones, and someone plugs in their laptop to the port on the IP phone, will Cisco NAC authenticate the laptop behind the phone or not?

Thanks!