Well, it depence on what you want.
Cisco use their Skinny protocol on there IP Phones, but……….. it is still a H323 device. The same go’s for the AVAYA phones.
All vendor use their on top protocols to activate there extra features on the phones.
The trick is to use the phones just as a H323 device. This means that you lose the extra phone features but you will be able to use the phone as it is a netmeeting device.
For the call manager go in to the device section and choose the h323 device option as endpoint and you can connect any device.
There is a pdf avalable on the Cisco site how o do this.