another problem with cell phone systems is that the tele operators take a HUGE part of the profit. I'll give you an example from how it is in Denmark at least... 6 kr ~ $1 is charged when you send an SMS to the system, from that you make about 50 cents - the rest is profit for the phone companies. If its just close to the same in the US then we would have to donate double the amount needed - right now $1000 a year to keep it running. No need to line anyone elses pockets if we don't have too.
The other problem is that the phone systems mostly only work with the country its established in, so all non-us citizens wouldn't be able to use it anyway... Nah, paypal is the best for small solutions