How to buy Thai SIM card to make calls during your stay in Thailand

The visitors who came to Thailand with your mobile phone I would recommend you to buy a Thai sim card for calling to your friends during stay in Thailand
There are 3 big company who provide the phone service in Thailand
1 is the biggest and oldest GSM Thai operator
2 DTAC
3 Truemove

For 3 of them seem to be the 1st one is the best and get more people that other ,however they have a huge compete among them so the price and the service cost is very cheap for ex. you can buy Thai sim at 50-100 baht only and you can buy a prepaid card for your mobile phone with the rate is about 300 baht per month
If you want to buy go to the shop with this logo and ask for the Thai Sim and the service