Open the Phone app
Tap the three-dot menu
Go to Settings
Tap Blocked numbers
Turn on Block unknown/private numbers if available
If your phone has Call blocking settings, enable Block hidden numbers
Use your carrier’s call-blocking service if your phone does not have the option
Install a call-blocking app that supports private number blocking
Contact your carrier to request blocking of anonymous calls
Use Do Not Disturb to allow only contacts if needed
Save the settings and test with a private call