I asked a similar question recently, regarding setting call limits. If I recall correctly, a2b can either support simultaneous calls on/off but no easy option to set a predefined amount.
Why not restrict the carrier doing the test to a max credit limit? Due to the nature of the way a2b rates calls (after the caller has hungup, vs realtime) there may be some overages, but at least you know what you are limiting your exposure to (financially). Think of it, if they are an established carrier filling 3 trunks they have asked to "test" would be easy. That equates to roughly 30,000 minutes/week (i dont know how long you plan to test) and that can be costly.
Just my 2 cents on how I wold look at it financially while staying within my understood parameters of A2B...
Hopefully someone else may have a better solution, technically.
