I've lived here a very long time. I top up an old sim card with my number that I've had forever, just to keep it alive to receive calls and for the occasional outgoing call. But 99% of what I use is data, so every year I buy a new secondary SIM with a prepaid annual plan on it, and I use that for everything data. 50gb of data for a month, fast enough for seamless streaming and video calls, or even hotspotting my laptop for the odd video conference in any area where the wifi sucks.
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You can take it to a phone shop with your passport and they can register it for you easily.
Also, if you want to keep a prepaid sim alive for a long time, then top it up 20 baht at a time. Every 20 baht adds 1 month of validity. So if you wanna keep your number but will leave the country for 6 months, put 20 baht on it 6x, and your number won't run out of validity and the number won't get recycled back into the system. You can check your validity, plan, credit, etc in the AIS app. 👍🏼