I do believe that "hang-up" calls to a boiler-room making outgoing calls as fast as they possibly can would have negligible effect.........in fact, unless the incoming call happened to catch a line that wasn't in use it would only get a busy signal and therefore require redialing.
I can't help but think this is a lot of hooey over nothing.
Technically, had the operatives just programmed computer modems to dial the numbers repeatedly or have a fax machine sending blank pages over and over again they could have more plausibly done this instead of just calling and hanging up which gives you no plausible defense.