My Gevy Sim on iPhone 4, 4.2.1 (8C148), Jailbroken with greenpois0n, BB 03.10.01, carrier: Simple Mobile.

The trick is not rebooting your device.

1. Enable FaceTime in Settings
2. Install MiddleWhat from Cydia (found in BigBoss)
3. Install Claro D.R. from Pushfix (source)
4. Disable FaceTime in Settings and Enable it back (wait for the message "Your phone number will be shared with people you call with FaceTime.")
5. FaceTime is now activated

Go to your contacts and select a contact, you will now see a tab for FaceTime

* YouTube Video coming soon.