Aryeh,
If you go to some of the various VoIP vendor websites you will find areas that have various success stories on who is using VoIP. There are several entities in the USG and DoD that are using VoIP such as USPS, Census, USAID, Army, Navy, Air Force and Marines are using VoIP to some extent.
Dow Chemical is antoher big global user of VoIP. Also your major long haul telecommunication companies are using VoIP for transcontential transport.