Just remember last year where I got a free Tourist SIM at the Immigration. Tried to activate the package which was shown at the flyer provided with the SIM, but all the True Move staff told me, that its out of date and impossible to activate it They forces me to buy a new SIM card from them for a lot more THB. I declined and toped up the free SIM in the Convenience Shop at the airport, after that I was able to get the cheaper package which was on the flyer. So they lied to me just to earn a little Bath. I guess thats Thailand, too.
So now I hope I can get a DTAC SIM at the airport, not a tourist SIM because the common packaged will fit better to my needs. If not I will try to get one in Krabi.