Robocalls seem to be an inescapable plague of the 21st century. But why? Raymond Huahong Tu, assistant clinical professor in machine learning, cybersecurity and computer science in the First-Year Innovation and Research Experience (FIRE) program, says it’s for a few reasons: They’re cheap, simple and potentially wildly lucrative for the scammers; it’s easy to evade the caller ID system; and not much can be done to stop them.

In an essay published Tuesday in The Conversation, Tu suggests the best thing for individuals to do is think about their phone numbers in the same way they think about their Social Security numbers: as private information to be given out only when absolutely necessary:

Of course, your phone number may already be widely known and available, either from telephone directories or websites, or just because you’ve had it for many years. In that case, you probably can’t stop getting robocalls. My advice for dealing with them is to stay vigilant. Don’t assume the Caller ID information that pops up for an incoming call is accurate.

You could, for instance, not answer the call and see if the person leaves a voicemail. Or you could ignore the call and dial the number it came from yourself—connecting you to the real person or organization the call pretended to come from. Lastly, if you do answer the phone, don’t assume the caller is telling the truth. Ask questions to help you determine that they’re legitimate—or not. And hang up if you have any doubt at all.

Read the full essay here.