AIS has this ever going promotion called "Mao Mao 30baht" which gets you 30 days validity and free call 30 mins for all network within 5days.
Key in *500*9# with valid credits inside your AIS sim card and u will extend your validity by 30 days and get free 30 mins call.
U get to extend your validity as well as calling worth 30 baht.
