Receiving FaceTime calls from unknown numbers can be both unexpected and disruptive. To help you manage these unsolicited calls, iOS offers a feature that automatically silences FaceTime calls from unknown callers, sending them directly to your recents list as missed calls.
Silence unknown FaceTime callers from settings
Step 1: Open the Settings app on your iPhone to access your device's configuration options.
Step 2: Scroll down and tap on the FaceTime option to enter the FaceTime settings.

Step 3: Within the FaceTime settings, locate and tap on the Silence Unknown Callers option to proceed.

Step 4: Toggle on the switch