Apple support's online article may help you: See how to: Get started with FaceTime on iPhone - Apple Support
Eg: says: Set up FaceTime
To get started with FaceTime, go to Settings > Apps > FaceTime,
then sign in to your Apple Account or turn on FaceTime.
If more than one contact method appears below “You can be reached by FaceTime at,” you can tap to select or deselect it. (More info. in the article if needed.)
More details and info. if needed: How to: Use FaceTime with your iPhone or iPad - Apple Support
Eg: says:
You can use FaceTime over Wi-Fi1 or over cellular on supported iOS or iPadOS devices.2
FaceTime isn't available or might not appear on devices purchased in United Arab Emirates. Group FaceTime and FaceTime Audio are not available in China mainland on iPhone and iPad with cellular.
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Troubleshooting if needed:
See what to do: If you can't make or receive a FaceTime call on your iPhone or iPad - Apple Support
or
If you can’t turn on or sign in to iMessage or FaceTime on your iPhone - Apple Support