Making anonymous or abusive phone calls is a criminal offence under section 43 of the Telecommunications Act 1984.
The actual wording is that a person who:
a) sends, by means of a public telecommunication system, a message or other matter that is grossly offensive or of an indecent, obscene or menacing character: or
b) sends by those means, for the purpose of causing annoyance, inconvenience or needless anxiety to another, a message that he knows to be false or persistently makes use for that purpose of a public telecommunication system, shall be guilty of an offence and liable on summary conviction to imprisonment for a term not exceeding six months or a fine not exceeding level 5 on the standard scale or both.
The above is a copy/paste so I can't vouch for its word for word accuracy, but you get the picture ! Contact the Police they will take the appropriate action.
One action to take, go down with some mates and make a point. Make a note of all phone calls from this IDIOT, "Do U know his/Her Mobile Number?" Phone Orange up and let them know what is happening, I think they can logo all phone calls made to the said mobile. Also the police. Good luck. Do U live near DERBY, I'll come with U.