To be more correct, dtac SIM card and use dtac Call app.
Once dtac Call app is setup, you don't even need the dtac SIM to be in the phone. As long as there is WiFi / data, dtac Call app will work like a virtual phone. Calls and SMS will go thru the app.
dtac app = check the status of your SIM details .. i.e credit, validity, etc.
dtac Call app = VOIP app. Calls and SMS charges will apply as if you are using the phone. App needs to be installed when you are in Thailand or use VPN.