With AIS no need for an app. If your phone has WiFi calls just enable it and ask AIS to open the service for you before you leave Thailand. When you arrive at your destination make sure to put your phone on airplane mode, then switch on WiFi, and you're good to go. The phone makes and receive calls as if you're in Thailand. Cost of the calls are according to your plan with AIS. However, of you make a local call on your destination - it is an international call.