How to stop harassing phone calls from Las Vegas number?

I kept getting calls from a number in Las Vegas (sometimes twice a day) so I blocked the number. Then they called from a different Las Vegas number and I blocked it. I’ve probably blocked at least four Las Vegas numbers now, but they keep calling with a new number. When I answered the first few times, no one is on the other line, it is dead silent. When I call back, it says the number is not in service.

How do I stop this without changing my number?