iOS users get to do voice & video chats with each other using the FaceTime. To enable / disable FaceTime on iPhone 5 and other iPad and iPod Touch devices, do as follows:
1. Open the Settings application
2. Scroll down to the list of default apps on your iPhone and tap to open the FaceTime settings
3. Simply toggle off the FaceTime button to disable FaceTime on your iPhone.
4. This action will make FaceTime no longer active and your contacts cannot reach you via FaceTime.
Note that, you can also completely restrict the FaceTime by doing as follows:
1. Open the Settings app and go to the General settings
2. Scroll down to the Restrictions settings and open it
3. If it is your first time using the Restrictions on your iOS device, you have to specify a new passcode, otherwise enter your passcode to open the Settings
4. Under the Allow list, find the FaceTime and make sure its disabled.
5. Go back to the home screen, and you should not see the FaceTime icon anymore