For International calls only, no need for a SIM. Go online and look up yolla.com. VERY cheap to call the U.S. or wherever. I can call Internationally from the U.S. too..and very cheap(not necessary that much nowadays if your friends/fam have the same Social Media website to call(FB, WhatsApp, Viber, etc).
Big financial meeting in Thailand in mid-Oct. Hope they lower interest rate. Would be smart for them to weaken the Baht some right before high season for tourism