Open Settings
Go to Control Center
Add Screen Recording if it is not already included
Open FaceTime and start the call
Swipe down from the top-right corner on iPhone with Face ID, or swipe up from the bottom on iPhone with a Home button
Press and hold the Screen Recording button
Turn on Microphone
Tap Start Recording
Wait for the countdown to begin recording
Record the FaceTime call
Tap the red status bar or recording indicator when finished
Tap Stop
Find the recording in the Photos app