I manage 2 small networks on 2 different continents and the 2 user groups need to coomunicate with each other through VoIP. I picked Skype becuse it's free, works fine and encrypts the voice communication. What I need to do now is preventing the users to spend the whole day chatting with their old school mates, so I would need to find a way to limit the Skype contacts they can use. Alternatively, is it possible to somehow limit the connections to the IP of the networks?
If this is impossible, would you recommend a valid alternative to Skype to reach my goals?