Does anyone know how to track down prepaid phone numbers?

Hello everyone, yesterday someone on WhatsApp has threatened to kill me. I already contacted the police and the respective service provider about it, but I will have to wait a very long time for results.

I know I'm grasping at straws here, but there must be a way to track down who was behind this right?

Or does anyone know a place on the internet where this would be an appropriate question? Thanks in advance.