VoIPover3G tricks applications into thinking that they are on a WiFi-Connection, even though they are on a cellular-connection like GRPS, EDGE or 3G.
This removes restrictions put in place by the developers, apple and AT&T and allows you to make calls using VoIP apps like Fring or Skype.
Please checkout 3G Unrestrictor the premium successor of VoIPover3G, which allows you to trick other applications like Slingplayer and many other easily..