Telemarketers have upped their game. They don't call from an 800 number anymore. They know you have caller ID. Instead you see a phone call coming in that looks like it's from a local number. Is it someone you forgot to store in your contacts?
Probably not. It's a fake number from a robo-dialer. It's called spoofing. And blocking that number is pretty much pointless since they'll just change the number.
Well the FCC is on it. They want spoof calls blocked via caller authentication.
Until then, if you see a phone number on your caller ID that you don't have stored in your contacts, it's probably best to just let it go to voicemail. If it's someone important calling, they'll leave a message or send you a text.
Or here's an idea too - change your number to an area code that you don't have friends in. Whenever you see a random phone number calling from the area code you've switched to, you'll know to just ignore it!