Sim card registration Deadline July 31
All SIM cards used in Thailand must be registered by the end of the month.
Extended stay foreigners may have registered their phones a few years ago when the SIM card laws were first announced.
If you are unsure, check with your network provider.
If in doubt "do it."
If you are out of the country until after the deadline you will have to register and 're-activate' your sim card in order to use it
Bring your phone and passport to your service provider ( AIS - dtac - True Move - etc.)
Tourists should not use the sim card from their home country unless they have thousand of dollars of spare change.
A Thai SIM card can be purchased for under 300 baht