>>>Surely the authorities can trace the phone numbers concerned and therefore the culprit? Or is it not as simple as that?
Many (probably most) pay-as-you-go phone users never register their SIM card with the service provider, so there's no way of linking the number to the user. A new SIM card typically costs just a quid from a supermarket, discount store or corner shop. Once you've got one you can top it up by buying a voucher from the same outlets using a (cloned) contactless debit card (so no PIN is required).