O2 numbers are released in running batches. ie 07877878000 -> 07877879000
Mr Scammer has to just go into an o2 store and a carphone at the beginning of the month, and ask to see what numbers are available, "becuase they want to choose a good one for their new contract". The scammer then knows that the next 5000 numbers or so, that are from carphone, and the same from o2.
The numbers get programmed into an autodialler machine..... RING RING - "hi, i'm calling from Scammers Insurance, you've recently got your phone from o2 (or carphone), but you haven't been given the correct insurance, it should be £99 for they year... blah blah blah etc... i just need to ask you some security questions, i can confirm some basic details with you to prove i'm offical, its an o2 line, on contract, less than £50 per month, your number is 07877878001, and its from carphone warehouse. If you could confirm with me your name, etc etc etc..."
Ofcourse there is a chance it is the shop staff, or indeed the organisation. But the shop staff would just sell on your personal details, no need to call you, and I'm certain CPW dont have insurance scams as part of its business plan.