TV shows (and radio shows) often need to feature phone numbers (when a character tells someone their number or something). The density of idiots, crazies and other subnormals in the world means that any number read out on TV is going to get dialled. If they used real numbers, there'd be some people getting pretty pissed off at being called by crazies. Therefore, OfCom have a list of telephone numbers for drama purposes. They've got a range of numbers in each major city, one for mobiles, and an entire fake area code.