How do spam callers mask their phone numbers to ones registered to someone else?

When you get a phone call and a number shows up on your caller ID, that’s not always because your phone knows which number is calling it. The phone that’s calling you can send the call and tell your phone what number to show, plus maybe a name as well. Scammers use a VoIP phone and can just alter the data that their phone sends to your phone, your phone doesn’t know any better and just shows what ever the other phone tells it to show.

Edited for clarity.

It’s called spoofing, and it actually exists for legitimate reasons. For example, a business with many individual phone lines may want them all to show up the same on caller ID so that customers call the correct number back. Or a person may want calls from their cell phone to appear to come from their office phone. Unfortunately now we’re dealing with people misusing this system.

It used to be somewhat complicated to spoof a phone number but these days it’s trivially easy. That’s because a lot of phone traffic isn’t actually done over traditional phone networks, it’s done over the internet using a protocol called voice over IP (VoIP), in which case all you have to do is send deliberately incorrect caller ID data.

You know what makes me mad about the whole thing is when you get spam calls from the same group ie: auto warrantees. You block the number that it came from and every day they call from a different number so each day the call goes through anyway.

This happened to me once. As in, my number was chosen to be used to attempt to scam someone. I only knew about it because the person being scammed called back and ended up having a go at me. It was a weird 30 minutes

Spoofing is when you are pretending to be a directory number that does not belong to you and masking is when you want outbound calls to appear from a different directory number of your org. Masking is done for a number of legitimate reasons.
Most carriers will check and enforce the format of the directory number but do not check if that number is yours. If carriers enforced not accepting directory numbers onto their network that don’t belong to the peering org we could eliminate a lot of spam and spoofing. They have the network resources to do this but it would add some overhead and cost and would require laws and regulations which isn’t easy.