Why do these exist? I've gotten three and it's really annoying why do I keep getting them?!

Also, I got a voicemail from someone claiming to be my ex's girlfriend (although I'm pretty sure he's still single) and saying if I don't back the f***k off then they were going to call the police and their dad on me?! If you go to my school and know anything about this, please explain this to me!!

UPDATE- I've gotten 5 messages now and I found out who called me. It was my stupid friend😂