Open your phone app
Tap the menu or settings icon
Select Call settings
Turn on Block unknown callers or Block private numbers
On iPhone, go to Settings
Tap Phone
Turn on Silence Unknown Callers
On Android, open the Phone app
Tap Settings
Tap Blocked numbers
Enable Block unknown/private numbers
Use Do Not Disturb to allow only contacts
Add unknown numbers to your block list manually
Contact your carrier for call-blocking options