When you arrive at Suvarnabhumi-Airport, you can get a free SIM-Card for your telephone / smartphone – if you whish.
You have to stay on the arrival-level and not take one of the escalators to the public transport level.
Then you see the telephone-company booths already.
The next telco is my favorite one. DTAC of Thailand. They are a little bit more expensive, but their Internet-Connection is super fast. If you owe a smartphone, this is for sure the perfect choice for you.
Do not forget to subscribe to one of their internet-packages. Otherwise you are charged by the minute, which is more expensive, as buying a weekly or monthly transfer-budget.
DTAC uses the frequency band WCDMA 850 / HSPA 850 MHz for 3G-services (HSPDA). Take care, that your mobile supports this frequency.
AIS is the largest telephone-company in Thailand. The internet-service is comparable with DTAC, so also very good – but expensive. But you would choose an AIS-SIM-Card, if you travel up-country and need a telephone-connection in even the smallest city.
As with all SIM-Cards in Thailand – you can use your internet-connection on a pay-per-minute basis (very expensive) or you subscribe to one of the weekly- / monthly-packages.
AIS uses the frequency band CDMA 900 / 3G 2100 MHz for 3G-services. Take care, that your mobile supports this frequency.
TrueMove is the Cheap-Charlie of all telephone providers. Their services are extreamly cheap (provided you subscribed to the correct plan) but they clearly lack the internet-speed, that is standard everywhere else in the world.
Unfortunately you even have to get a “special” SIM-Card for their True-HSPDA-Services. The SIM-Card, they are giving away for free in this booths will not work with 3G-speed.
TrueMove uses the frequency band 850 MHz for 3G-services (HSPDA). Take care, that your mobile supports this frequency.
On the other hand – if you use this SIM-Card only for local- or international-calls in Thailand, TrueMove is a cheap alternative.