AIS One-2-Call has many many plans, usually higher the per minute charge longer the validity period. For examples, the Sawasdee Plan charges 2 Bahts per min and a 100B top up will last for 30 days, but the Toh Sabai plan charges 1.5B /min and needs 300B top up to last 30 days. One can switch plans by paying 30B each time.
One-2-Call! : Call Plans For Current Customers