Firstly, if you have their number then they are really bad at prank calls... *67 all the way... Not that I would know... Moving on...
Secondly, if you have a number, and it's really, really bad prank calling, like, won't stop, innapropriate, etc, you could always call the cops and give em the number... they'll track the source pretty quick...
Yea, I know that doesn't help too much with the, "finding their name" (in essence, your entire question) but idk how to do that. What I told you is all I know about the topic. Sorry I couldn't be of more assistance.