How to find out who called me?

A private number kept calling me earlier and saying they are in my attic and that I'm going to die in my sleep. They said they're my worst nightmare and other stuff like that. I know it's fake but I want to find out who it was or what their number is. Is there any FREE way of doing this? Btw I have an android so I can't get apps that are only on iphones

You must not understand what private means. Never answer a private number, what makes you think you always have to answer? Put it on mute when you go to sleep.