If you are one of the lucky ones who got in on the GrandCentral beta, then you might be interested in a new iPhone app that integrates with the GrandCentral service. Grandialer (iTunes link) allows you to use your iPhone to call people using your GrandCentral telephone number.
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.