This is only relevant for people on long stays. I've had a monthly prepaid data plan (mao mao 450) for a long time now. The way it works is that if you have sufficient credit the plan will automatically renew and if you don't you're just cut off from your data-plan. Not anymore. This month, although my balance was enough for the plan to renew AIS started charging me 50baht a day for data and before I realized it drained half my credit. When I went to the AIS office the girl explained that t