couldn't you make a method of purchasing credits through sms (like prizee has, for example)?
for orange and vodafone users who maybe don't have credit cards it would be an alternative and a way in which they could help too (especially for students whose phone subscriptions are being payed by their parents).
i don't know if this would help in any way, i suppose you'd have to make an agreement, a contract or smth with orange and/or vodafone for that but there would probably be profit too... i guess
anyway, it's just a thought.. just trying to help. i'm sure things will turn out allright in the end because the game is too amazing and there are too many people supporting it and who want to help so nothing can possibly go wrong