The last time I was in Thailand in October I picked up a free sim in the airport with True which gave me 4G internet and calls in and around Mukdahan in the North East. I ended up doing the same for the GF before I came back because her DTac sim didn’t have 4G where she was and I can put money on her sim with a CC from the UK with no problems. 4G was way better for Skype etc. New cell towers have popped up all over the place in Thailand since my first visit last February and it is a lot better than it was.
Somebody mentioned on here a while back that you need a passport to buy a sim but I didn’t find that although they mentioned that it is a new law in Thailand.