Why my iphone X keeps on restarting during the phone calls

Perform a "forced" restart of your X, using these instructions:--> Force restart iPhone – Apple Support

If the issue persists, it might be a hardware problem (digitizer?), but we, just users like you, cannot diagnose that kind of issue remotely.

Contact Apple Support directly (they have tools to run remote diagnostics) using this link:--> Get Support

or have your iPhone inspected at an Apple Store or Authorized Service Provider.

Regards

Giulio

Yes, do a hard reset as suggested by Giulio. If that doesn't work then make a back up of your phone and restore it to factory defaults. After that set it up as a new phone and add all the apps you need manually and not from a previous back up. This should solve it, hopefully!

Regards