To be vulnerable to the 25 cent charge, you have to make a phone call. Why is anyone setting up a bot to make huge number of phone calls? The phone system is meant for people to make phone calls to other people.
If actually a problem, having the charge go to charity rather than the recipient would eliminate any monetary incentive. I suppose one is left with someone who has a grudge against AWS trying to bankrupt them 25 cents at a time...