TheFaQQer - scorchio!
I live on CUK
NickFitz - scorchio!
However, it's also very easy to find out which telco owns a particular range of codes right down to (the modern equivalent of) local exchange level, and is therefore most likely to own a particular number, nowadays.
For example, my home phone number is in the nnnn-nn range (where n is an integer between 0 and 9), which immediately narrows it down to ntl Group Ltd, now part of Virgin Media Ltd.
There's an iPhone app called "UK Numbers" that narrows down the search as you enter the number. But you can find the same information, but more up-to-date (in that it realises that ntl is now part of VM) via this useful phone number lookup web application. For example, a search for 01793 will give you Swindon; a search for 01793 59 will tell you that numbers within that range are mostly owned by BT.
However, be aware that number portability means that an individual phone might have a number within that range, but ownership of the subscriber line has been transferred by a disloyal customer to another telco such as Telewest (now part of Virgin Media, but no better for that).
pmeswani is too good to be a permie
Try calling 150 and see what happens... or even better... try calling 100 and see what happens. You may find out who owns the line via one of these routes.
If your company is the best place to work in, for a mere £500 p/d, you can advertise here.