Thanks for using Apple Support Communities. I can see that you want to use FaceTime over cellular data. Although adding an email address to work with FaceTime can help give others options to reach you, this will not allow for cellular data to work. However, I've included a link below with some information of settings that need to be confirmed to help with this.
If you can‘t make a FaceTime call on your iPhone, iPad, or iPod touch
Specifically, this section
If you can't make or receive FaceTime calls
If none of the above reasons apply to you, and you still can't make or receive FaceTime calls, follow these steps:
- Make sure that your device has a Wi-Fi connection to the Internet or a cellular-data connection.
Learn more if you're using router, firewall, or security software that restricts Internet access.
- If you're trying to use FaceTime over cellular, make sure that Use Cellular Data is on for FaceTime. Go to Settings > Cellular > Cellular Data, and turn on FaceTime.
Let me know if this doesn't work for you.