[maroonblazer]

Had no idea this setting existed despite being an iPhone user for years. Thank you!

Settings->General->Phone->Silence Unknown Callers

[WinstonSmith84]

For Android, it's directly in the Phone App. Then, the 3 dots menu top right > Settings > Blocked Numbers > Block calls from unidentified callers

[victor106]

On my version it’s here

Settings->Phone->Silence Unknown Callers