This is an answer that we have been seeking for some time now and cannot find an answer to. The only thing that we have found so far is if you go into your IIS logs on your exchange server you will find a separate entry for the phones that are connecting to your server. Each entry will start like this:
running a quick script against that should be able to get a list of all users. I am still looking for a quicker web-based tool, but for now that is the hard way. Hope that helps.