Also it could just be a pay as you go phone which has not been registered. Its quite common for criminals to use these phones as the numbers are untraceable to an address or person.
You could report it to the police as it could be classed as menacing/abusive so they have a log of it. You could also report it to the network O2 or vodaphone etc depending on the number - the police will be able to tell you or if you put the prefix on here someone will be able to tell you who the network operator is.