If you want to make calls with your Thai SIM card, you need a credit. And one that hasn’t expired yet. This is because each top-up has only one duration or validity period set by the telephone company. So if you are in Thailand for a few weeks in winter and want to use your SIM card for your next trip to Thailand, you must make sure that you have a remaining credit with a sufficient validity period. I will show you how this works here.
Usually you will receive additional validity days with every recharge. Currently there are 30 additional days of validity after each recharge of any amount. So you can easily extend the term of your remaining credit by recharging your Thai SIM card with credit several times.
To check the remaining credit on the SIM and its duration, you can use these so-called USSD codes of the different telephone companies.
AIS: * 121 #
DTAC: * 101 * 9 #
True: # 123 #
With the following possibilities, the duration can be extended:
1. Buy 10 Baht (or any amount of money) credit at a machine or in a shop brings 30 days extension.
2. A method I have not tested: * 500 * 9 # to get 30 day extension for 30 Baht via Mao Mao package
Extend your validity for up to 365 days at a price of 2 Baht per month with the following codes:
* 113 * 30 * 9 # (30 day extension for 2 Baht)
* 113 * 60 * 9 # (60 day extension for 4 Baht)
* 113 * 90 * 9 # (90 day extension for 6 Baht)
* 113 * 180 * 9 # (180 day extension for 12 Baht)
Condition: Activation of the SIM card for at least 90 days or use of 200 THB.
With “Quick Add” you extend the validity of your remaining credit up to 365 days at a price of 2 Baht per month
* 934 * 30 # (30 day extension for 2 Baht)
* 934 * 90 # (60 day extension for 4 Baht)
* 934 * 180 # (180 day extension for 12 Baht)
I have been using AIS for years and can confirm this approach. For the other two Thai phone companies I have the information from the English website: http://help.mobiletopup.com/knowledge-base/how-do-i-extend-the-expiry-date-validity-of-my-sim