You must have a jailbroken iPhone to follow these instructions.
Press to launch Cydia from your Springboard.
Press to select the Sections tab at the bottom of the screen.
Press to choose Tweaks from the list of Sections
Tap to select MiddleWhat from the list of Tweaks
Press the Install button at the top right of the screen.
Press the Confirm button to begin installation.
Press the Restart Springboard button once installation has completed successfully.
Tap the Settings icon on your Springboard once it has restarted.
Choose Phone from the Settings menu to continue
Ensure that FaceTime is turned on!