In the Philippines, we prepay the load for the mobile phone. We can charge the load everywhere in the city, and each one can decide the amount.
In Japan, fixed monthly payment is common. The monthly load volume is decided according to the contract type, and the surplus amount is carried forward to the next month.
On the other hand, if you run out of it you can add for a fee. But it is expensive. So it would be better to wait for the month to change if the end of the month is close. You can use e-mails and SNS, since communication at low speed is possible, even after you run out of the load.