From Skype:
Skype for Pocket PC is free and simple software that enables you to make free calls anywhere in the world with your handheld. Created by the people who brought you Kazaa, Skype uses innovative P2P (peer-to-peer) technology to connect you with other users. It is quick and easy to install and offers several features, including SkypeOut calling from Skype to regular and mobile phones worldwide, free conference calling, and secure file transfer. Skype calls have better sound quality than your regular phone and are highly secure with end-to-end encryption.
