𝘕𝘰 𝘱𝘩𝘰𝘵𝘰𝘴 𝘴𝘰𝘳𝘳𝘺

If the button for FaceTime is greyed out, did you check if FaceTime is supported for your region?

Wireless carrier support and features for iPhone - Apple Support

Any restrictions set up for FaceTime in the settings for Screen Time, or is there a profile installed and this is a managed device?

Set up Screen Time for yourself on iPhone - Apple Support

Also have a look at the troubleshooting mentioned in this related article:

If you can't make or receive a FaceTime call on your iPhone or iPad - Apple Support