Basically from what I can gather, this number is used by a company, who appear to be Comms Direct,
http://www.commsdirect.net (although they MAY be just hijacking a good firms reputation, I don't know as I don't know the company).
Anyway, they phone people on ALL networks who appear to be Pay as you Go customers in a bid to offer you a better deal.
Using your phones prefix 07633 for example, they can usually accurately guess what network you are on so can appear to be part of the company without actally saying they are, note that they will not know who you are and will not call you by name, (unless they have rung you previously).
Anyone who gets fooled into this will eventually be passing over their bank details to get signed up to a new 12 month contract with some random comms company you've never heard of, you may end up with a better deal, but I HIGHLY doubt it!
A company, with terrible practices and a VERY poor phone manner - Not good for a communications company!
Hope this helps!
TIP: If they ask you how much you spend a month on PAYG tell them £2-5 quid, that tends to send 'em packing!