With the help of VOIP you can replace your old style phones and upgrade to a more advanced form of communication.
Benefits:
-lower prices
-more flexibility
-better sound quality
-instant messaging
-video conference
-and many other things to improve the way you communicate

Tools you need:
-softphone
-voip server

Learn more about VOIP:
http://www.compare-voip.net/articles/index.php
http://en.wikipedia.org/wiki/Session_Initiation_Protocol
http://en.wikipedia.org/wiki/VoIP
http://www.voip-info.org/