See If you can't make a FaceTime call on your iPhone, iPad, or iPod touch - Apple Support
Check that cellular data is enabled for FaceTime
If you're trying to use FaceTime over cellular, make sure that Use Cellular Data is on for FaceTime.
Go to Settings > Cellular. Find the FaceTime app and turn on Cellular Data.
Reset your Network Settings
Tap Settings -> General -> Reset -> Reset Network Settings. This also resets Wi-Fi networks and passwords, cellular settings, and VPN and APN settings that you’ve used before.
Check your contact information
Go to Settings > FaceTime > see what is checked off under the You Can Be Reached By FaceTime At. Restart the device.
Reset all settings
If the issue persists, reset all settings: Settings > General > Reset > Reset all settings
Contact Apple Support
If it is still not working, contact Apple support here Contact - Official Apple Support