Sawadee...
I know that AIS uses many phones....The only big deal is if your are a GSM customer (like me) then you need a high mghz receiver..
Actually my rate is .34 USD outbound and free inbound in Thailand but ...
I had a phone unlocked in April....600 Baht and then purchase some 1-2 Call cards and off you go...

I suspected that I just bought a SIM Card got a local number. I was a favor for a Thai business person that I know and they did have to show an ID card.

I sould be supprised, with all the mobile shops, if a vendor would turn you down.

Bottom Floor of Big C has about 20 mobile shops.

Don't for get the charger for Thai volteage...I got a charger for a Nokia 8290 for 300 baht and a 600 baht 1-2 call card at about 6 baht per min...

Just call a certain number on your phone and punch in the scatch off number and you've got minutes............