You're not treading lightly, are you? ;)
Unity has no APIs.. but since Unity is integrated into Exchange, you can access exchange to perform the deletion. It will require some extensive reading up on the Exchange SDK. I suggest this as a starting point: http://msdn.microsoft.com/library/default.asp?url=/library/en-us/e2k3/e2k3/_exch2k_searching_folders_http.asp (as it happens, the client who ordered the personal directory lookup suddenly had a change of heart now and now I have to access Exchange.. so that's how I got that link). Prepare to enter the world of WebDAV. There are other means, but WebDAV is the most modern and multi-purpose (as in you can use it from non MS environments using non MS programming languages).
As for third party systems.. you have to read their documentation and figure out how they work.. keep in mind that in the Cisco world, VoiceMail and CallManager are very loosely connected.. it's not like you can tell your callmanager to list available voicemails and the likes.
