This can be tricky but needs to be done if you want to recieve facetime calls to a new number.

1. Ok first go to Settings > Phone > My number. The new number must be in international format.
2. Switch off FaceTime in Settings > Phone
3. Make sure you can send and receive text messages.
4. Go to Settings > General > Reset > Reset All Data - this will reset the phone setting And will forget your wifi details.
5. Reset the phone (long press of the off button)
6. Reconnect to the wifi
7. Switch on FaceTime

Job done!