DON'T DO IT!!
The premium rate numbers will cost a fortune - we had someone try something similar to book accomodation.
I e-mailed him back and asked for the details of the people staying, and (giving him the benefit of the doubt) treated them like normal punters.
Then, as they were foreign 'executives', and were restricted in what they could pay for - they asked us to pay some of their UK costs (hire cars etc) and charge it to the credit card. We were asked to pay these costs by bank wire (!!).
I e-mailed him back, and suggested that he probably shouldn't be using that hire car company as they sounded dodgy, and we couldn't take anything but the accomodation costs - and, guess what - never heard from him again!!
If you get taken in, I suspect what they do is give you someones stolen credit card details, which go through absolutely fine. You then wire some funds to an account (bank wires being conveniently untraceable/anonymous), and charge that to the credit card as well. It all goes through fine.
THEN - the person who's credit card it is cries foul - and the card gets charged back to you. Scammer has his bank wire, innocent credit card holder gets his money back - YOU are out of pocket.
We get 3 or 4 offers like this each week at the B&B - we go far enough down the line with them to establish whether they are dodgy (occasionally they aren't) - and pull the plug when they ask us to do something we aren't happy with.
Hope this helps!!