ABAKUS

The service is included in a mobile version of Microsoft Office Communicator due to be released this year. It will take the form of a voice-over internet protocol (VoIP) application that allows Office users to make free voice calls over wi-fi enabled phones running Windows Mobile software. It uses the internet as a virtual phone network as well as accessing e-mail, PowerPoint and other Office applications

Also ein voll in Office integriertes VoIP Tool das wenn man mit Wifi eingeloggt ist eben gratis funktioniert...

mehr dazu bei The Business Online (in englisch)