Since I have an international roaming option on my mobile plan in the US (with T-Mobile), I've avoided changing SIMs and getting a Thai phone number on past visits. This time I decided to experiment with getting a Thai number as well since my phone (and most phones these days) support multiple SIMs. No more having to physically change the SIM when you want to switch between numbers.
So I went to the dtac counter on the lower level of BKK and had a dtac eSIM installed on my phone. It cost 699B for 30-days service (with some reasonable GBs included and a paltry call allowance). After 30 days you can load more money onto the phone at the 7-11. Well, after the 30 day period was up, it seemed no matter how much money I loaded onto the phone, it quickly drained away after very little use.
So what I learned is you MUST choose a new subscription plan for a fixed amount and not allow the company to charge you "a la carte".
Now there is also a gotcha with this as well: If you don't pay close attention to when your current plan is expiring, it reverts immediately back to "a la carte" and oops there goes your balance down down down again.
However when you choose a new plan period, the new plan does NOT add on to your existing plan, but rather WIPES OUT the old plan (and any days/GBs/call allowance which is remaining) thus you LOSE the remainder of your old period. Very sneaky and very unfair. Be very careful! I don't know if all mobile companies (AIS, true, whoever) do this robbery too. Does anybody know? Next visit I will not be using dtac (which is in the process of merging with true anyway).
I hope this post saves somebody some aggravation.