My house phone number is withheld, but if I want to show my number, what code do I dial before phone number?

Question

My house phone number is withheld, but if I want to show my number, what code do I dial before phone number?
A relative has barred withheld numbers and I need to show my number to be accepted to get the call through, but I cant remember the code to use before you dial the number, Does anyone know? Thanks

Answer

U.K is