Here's how it works.
The production company, or TV channel will contact a a firm like the one I used to work for. Basically they will build things on the end of phone lines, usually premium rate. So that includes ringtone services, competitions in your local paper, vote lines on BB, anything you can think of. These firms usually get their telephone numbers from a large supplier such as BT, Cable and Wireless etc. BT will give them a �1ppm number on a revenue share basis and probably take about 18p, leaving the company with about 82p per min from it. They will come to some sort of contract with the production company to offer them a revenue share scheme as well. The bigger the show, the more money they will probably hand over. I'd be willing to bet that The Mint probably take upwards of 75p per min, and the telephone company might make about 5p per min.
As the production company don't run the lines, they can't just stick their researchers in, not without a lot of hassle anyway. Far easier for them to say to the telephone company, 'send us a caller through in 10 minutes and turn down the rest that call'. So all callers will probably be real, but your chances of getting through are very slim. With repeat callers they might take 200,000 a night, so you can work out the odds for yourself.
Low production costs and 200,000 calls at �1ppm every night equals profit making TV.