The Do Not Call Registry is a service provided by the FCC and allows anyone with a phone number to sign up free of charge. Unfortunately, only reputable companies comply with this list. If you've signed up but continue to receive spam calls, it's because the numbers reaching out to you aren't legitimate businesses. Another unfortunate aspect of spam callers is that there isn't a way to block all of them. While you can stop the numbers individually, many spam calls come from auto-dialers and systems that regularly create new phone numbers or fake existing ones. This means that you'll continue to receive some calls regardless of the precautions you take.