I would definately see the benefit of using a service like this.
I heard of one place that charged £10 per hour of phone calls, plus a bonus of £15 per appointment made.
So if the person spent a whole day (say 6 hrs) doing calls and you got 4 appointments from it, the cost would be £120, which is a bargain, because if your any good, you should be able to get a good proportion of the appointments as customers. This would be for commercial window cleaning only by the way, this wouldnt work for domestics i dont think.
I think that woud be acceptable to pay. The thing is too, you cant just do it for one day, and if it doesnt work say "this person is rubbish". Some days will be good, some will be bad, thats just luck. I think you'd probably need to comit for a 3 or 4 days to get a good measure.
Just for the record kate, i really dont think you should ever do free trial. People will massively take advantage of that, and the service your providing is valuable.