Settings, Facetime, turn facetime off
Restart the phone
Settings, Facetime, turn it back on
If no better
Backup, restore to factory, and restore from backup
If you can import your media to Photos or iTunes in a computer and copy important documents to a cloud server it will give you some resilience in case the restore from backup fails.
(1) How to back up your iPhone, iPad, and iPod touch - Apple Support
(2) Restore your iPhone, iPad, or iPod to factory settings - Apple Support
(3) Restore your iPhone, iPad, or iPod touch from a backup - Apple Support
If the problem remains then do step (2) again but set up as new without restoring any backup. If that fixes it then it is probably a rogue app, probably security app..
Otherwise arrange a repair
iPhone Repair - Official Apple Support