We have tried them all and we were doing some heavy weight calling averaging around 24 hours per week, Mick is right certain telesales outsource their work and can not keep their eye on it, what you have to do then is request a recording of the session, to see how they perform, you will laugh at some of them when their calling the person the wrong name.
Mick is correct with some of the appointments they will say yes for an appointment just to get you off the phone, this then is just wasting your time to visit a site and they inform you they are comparing prices, my advice would be to employ someone local and do it in your own house, and state how many calls you would expect per hour.