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There is a reason they don't give contracts to minors. That aside, if he is using over his allowance then the best thing would be to increase his monthly package. While that in itself would cost you an extra few pounds each month it would save you the £40 bills. Charges outside allowances are often extortionately priced so having more mins/texts would save you a lot of money. Go through his bills and see how many mins/texts he is using each month and work out the best deal.