system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: How do spam callers mask their phone numbers to ones registered to someone else?
gpt
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.