Data voice calls app Does a free app exist in the UK which can be installed on two android phones allowing you to make free data voice calls over an internet WiFi connection for free without having to register a new account with the app installed?

For example, with Skype, you have to register a Skype account before you can start using the application.
You can try viber. You only need to put your phone number.

Personally I don't like it because of the UI and their notification mechanism but its a matter of personal flavor.