Ip telephony has really taken off with some superior cost savings for big corporates.
There are many horror stories around VoIP and IP telephony deployment. There are also many successes. We have just started working with Integrated Research with their product Prognosis. The thing about VoIP deployment is that a CIO needs to know whether or not their network can handle the new bandwith requirements with all the phone calls. The other major problem with deploying VOIP to a corporate network is to try and monitor it and have visibility over where something is going right or wrong. These guys can tell you heeps more about IP telephony monitoring and how to ensure that your whole phone system doesnt just suddenly crash.
Network assessment is a key component of this, but is not enough on its own to ensure VoIP success. Id also think about the demands of data, voice, and perhaps video on your network.
I think this is very important for anyone who is rolling out a system with more than 5,000 handsets.