Open the Phone app. Tap Settings → Caller ID & Spam. Turn on Filter spam calls. Go to Settings → Phone → Silence Unknown Callers. Calls from people not in your contacts will go straight to voicemail.

Cell phone spam is a real problem. In fact, one tech analyst writing for Forbes recently opined that the “Do Not Call List” doesn’t work and estimated as many as 100 billion robocalls—calls lacking a ...