A difficult question and one that you really have to answer on your own, but how badly do you need to know who made the call?
If you have been getting prank calls then you can just report them to your local police station and then it is in their hands. If you trust them to pursue it then that should be the matter finished. If you really want to be certain that someone is going to take action then you need to get in touch with their phone provider. The service provider has the power to do a lot more and it could lead to the person making the calls being disconnected.
You will really struggle to phone their service provider though without the use of a reverse cell phone service as without them then I don’t know of any other way of identifying who they are with (unless you phone their voicemail and they haven’t changed their message then sometimes you can tell from that). So the best option is really to use one of these sites and dig up the service provider and the details of their name and address.
Now you are better equipped when you want to go to the police and report them as you can give their name and address, of course you should still go to the phone company too.
That is a double-whammy that ensures the person gets into trouble. That doesn’t necessarily mean the person will stop, they can get a new phone to call you from etc - but having their name gives you an advantage and if you let them know you are aware it is them then most people should get the picture and it will not seem so ‘fun’ when you know who it is.