A lot of different companies offer telephone shops. I don't know of any that specialize in them. As with any type of mystery shopping, the more companies you register with the more opportunities. You will actually have to go through the process in order to access their job boards. The official list at the bottom of the page is a great place to start. Bestmark, Corporate Research International, Measure Consumer Perspectives, Market Force and Marketing Endeavors are some that offer phone or internet shops. I'm sure there are many more out there.
Equal rights for others does not mean fewer rights for you. It's not pie.
"I prefer someone who burns the flag and then wraps themselves up in the Constitution over someone who burns the Constitution and then wraps themselves up in the flag." -Molly Ivins
Never try to teach a pig to sing. It's a waste of your time and it really annoys the pig.