The service can be used on EDGE since it's not a VoIP service. The application works by connecting your calls through GrandCentral and ringing back your iPhone. To set up the application, you just need to specify a ringback number for GrandCentral to call you back. To do this, just navigate to Settings > Phone number in the Grandialer application.
Grandialer is a free application and is available today on the App Store. For more information on the application you can visit the developer's site.