Is the number appearing on screen when the phone rings ? you might be lucky - unless it is a pay as you go.
Pay as you go can be bought on car-boot sales for cash, so no chance there, but a contract phone is registered to someone, unfortunately unless the nuisance calls are threatening there is not much an operator will do.
If the calls are threatening, tell the police and give them the number that calls, or if your phone is pay as you go, change the number.