In this days, micropayments are very popular, for people that is 14-17 Year old, they don't got any Credit Cards, so they wont be able to donate. BUT
By making a Phone Bill Way, it will increase Server Income.
Here is some of my Suggestions for them.
1) ZayPay (Really Good service, but price margin is not 100% same on different countries. You need at least 50$ To Withdraw to your PayPal & Bank Account.)
2) BoxPAY (REALLY Nice, Stable, LIVE Help by they just call you and helps you, and NO i don't know how much you need to withdraw to your PayPal)
3) OneBip (AWESOME Service, but the Amount minimum amount to withdraw is 200? which will take FOREVER)
4) Fortumo (Nice and easy to use. Minimum amount is 50$ to Withdraw. It got more Systems than other Micropayment Websites)
5) PaySIMPLE (Very nice, easy to use, minimum amount 50$, but they bill 60% to themselves, and you get only 40% of the money your donators donate's)
NOTE: ALL Micropayment websites i Listed above, are taking 30% - 60% Charge automatically on every Payment, because money is not transfered as you think, it transfers from one place to another and on every place, it get's billed for them, and the Micropayment Website Staffs, take from that a little amount to themselves.
However, i recommend it

Will Increase Donators A LOT.
Regards!
//CyBeRooT - Muhammed Gultekin