This is a tricky question that nobody can really answer for you, but how badly do you need to know who owns that number?
If you have been getting prank calls then you can just report them to your local police station and let them take care of it. 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 police may not have the power to do anything, but their service provider should be able to do much more and it could lead to the individual making the calls being cut off or worse.
If you decide to go down that route and call their phone provider then you need to use a reverse cell phone website as without them there is no way of knowing who they are with (unless you phone their voicemail and they haven’t changed their message then sometimes you can tell from that). Even though there is a small charge involved then you can’t argue that the easiest option is to use one and find out the service provider and the name and address of the person.
Now you are better equipped when you want to go to the police and report them as you can give their name and address, then just to make sure 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 will be too cowardly to carry on when you obviously know it is them.