I read something about having to register the sim card.
Will the store employee help me register it, or I have to do it on my own?

How much does the pay-as-you-go plan cost? I only need to make very few phone calls. I don't need any data.
Thanks

Just go to a AIS or dtac shop. They will help you through the process. Plans are cheap, and easy to top up.

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^^^^ Don't forget your passport.

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All the major carriers, AIS, True and D-Tac have kiosks in the arrivals hall of the airport..open 24/7

They will set it all up for you.

D-tac usually has the shortest line.

 

If you only need to make a few calls, download a voice messaging app such as Line or WhatsAP or even Facebook.  You'll be able to make calls via WiFi, free at most hotels and a lot of restaurants and coffee shops...no SIM needed. The parties you are calling will need to download the same ap.  If you download Skype and put some money on the account, you can make calls to dialed numbers.

I would pay for some phone calls and SMSs also,  people will want to call or text you to a regular Thai number and its dead cheap anyway .......and you can reload at 7-11