So that the Thai military authorities can snoop on your communications, particularly your internet use, a new law has been decreed requiring mobile phone users to register their SIM cards with the authorities.
The new law will become effective on 1st February and requires that SIM users register their name and ID Card or Passport details by 31st July 2015. After that date they will not be able to use their SIM.
If you are a foreign citizen resident in Thailand, you will need to take your passport to register at any of the subscriber information registration points setup at Big C and Tesco Lotus, as well as 7-11 stores.
Last year, the independent SIM card resellers were told to use a mobile app to register the user SIM cards. Using the app, the salesperson would take a picture off your SIM card and your subscriber ID and it would be sent to NBTC for monitoring, both for new and existing users to register their numbers at any shop with the 2-snap logo. However, the app (which could be downloaded by anyone) was exposed to have lots of security issues.
Tourists who come to Thailand for a holiday and purchase a prepaid SIM card will need to show ID at the point of purchase in order to qualify as registered . This means a passport (valid for at least 6 months, which it should be anyway if you just travelled here) in order to register your SIM card.
Even if you have previously provided such information when you bought a SIM card in Thailand, it’s highly likely that you are not registered because the mobile operators seem to have completely ignored or failed to implement the proper registration procedures. How to register your prepaid SIM in Thailand