There are people working on commission who will trawl through classified ads for cars, vans, etc., for phone numbers. They will call up and promise all sorts of guarantees just to get their commission.
You should not part with money to a company that you've never heard of (however long the scam has been going) on the basis of no more than a single phone call.
I've found our local paper to be quicker but more expensive than autotrader.co.uk but, if you've not heard of someone, they can hardly be very good at getting the details to buyers, can they?