Almost everywhere in Thailand you have good reception of mobile phones. The roaming costs can be substantial.After my first trip to Thailand I had a phone bill over 100€ on my Belgian phone. That mistake I will never make again. If you want to be free from roaming costs in Thailand, you can take an old phone with you and buy a local sim card. You can buy it at any 7/11 store or Family Market (Ask for a Happy sim card) . In order to buy a local sim card, you need to register it with ID, and you need to activate the card following the instructions (also provided in English) that come with the sim card.
I always use HAPPY (DTAC) and I am happy with it. The cards come in all kind of prices, between 49 THB and 300 THB. The difference is the prepaid credit and the name on tha pack. My 300THB "tourist sim card" only had 100THB calling credit. So ask for the different sim cards and eventually buy credit separately. You sometimes get 7 days free Internet and Happy will send you messages all day to make you spend more. But hey, for 10-15 Euro per 2 weeks, I have internet, I call for taxi, make reservations at hotels and can call and SMS my friends in Thailand.