It is not possible to trace a phone handset to its owner (except for the police in criminal investigations), but you have done the right thing contacting the police. You should also phone your mobile phone service provider, and speak to their malicious/nuisance calls department. They may be able to give you help and advice.

Unfortunately, PAYG mobiles do not need to have a name and address registered against them (although there are proposals for new laws to do this), so, even if you have given the number to the police, they are unlikely to be able to do much - although if they really wanted to they could request location information from the service provider of that number for each of the calls which may help in finding the person

You can't trace it, but you can use a number change to find out who is stalking you.

Get a new number and give it to one person, telling them that you aren't using your old number. If after a few days, there aren't any stalking calls, give it to a second person. And so on.

After a while, you'll start to get the stalking calls on your new number, giving you a pretty good idea of who is responsible.

Get another new number (after all, you can get free PAYG sims from the net) and give this new number to them. If you get the calls on this number, give the details to the police. Even if this person is not the one making the calls, at least you will have found out who is giving your numbers to the creep and the police can take action against them!!

If the owner of this pay as you go phone has added credit electronically (using the credit card like phone identity card) and paid using a credit or debit card the police can trace the owner because there will be link between the number being credited and the 'plastic' used to top it up. I know this for a fact because it is a commonly used technique police crime intelligence analysts use to trace owners of pay as you go phones. Unfortunately, if top ups are by cash payment then tracing ownership is far more difficult, but where the top up is made using the plastic id card for the phone it is still possible to identify where the top ups were made.