Get a AIS Super Social sim prepaid. ฿200 a month. Then top up in ฿200 increments x12 for one year service, can be done in perpetuity, though only for 12 months max…so nearing the end of the 12 months you can top up again in ฿200 increments not exceeding 12 months. Before leaving the country, be sure there’s not a negative balance. While out of the country, providing your phone has WiFi calling capabilities, put the phone in airplane mode with WiFi on and make free calls to TH.