It's more than likely a robo-dial caller calling the numbers in your area code in numeric order. Regardless whether it's a telemarketer or scammer, the best bet is let any unrecognizable phone number to go to voice mail. If no voicemail is left, block the number. However, don't expect robo callers to stop anytime soon.
*Please keep in mind if you have children some school systems and hospitals calling will sometimes show as "No Caller ID" or "Unavailable".