I will recommend you to use Lebara Mobile.
You will then get a new sim-card and can call to Thailand for 6P/Min (mob/mob). I don't say it is the cheapest one, but it is very easy to use without having to dial pin codes and you can also charge your account on the net.
My wife use this phone every day for calling in Norway and to Thailand, as it is nearly the same price wherever you call in the world.
International Calls - Lebara Mobile
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