I’ve extended a few time over the years. Extending after the first time takes about 10 minutes work for the officer. You wait much longer in line than the actual work ;)
That’s why you can do it on the last day (which is not advised if you’re not 100% prepared).
Hahaha! Must have been the last booth north of Bangkok. They got me twice there. Since then I’m taking the leftmost lane leading to another booth apart from the main.
1. You don’t need to do anything if you’re willing to pay for roaming.
2. Get an AIS (best coverage) SIM card if you’re a tourist.
3. For long term stay either convert your home SIM to eSIM or get a cheap phone to receive SMS.
For the US there’s some providers of VOIP that support receiving SMS too.
Through using “wifi calling” (turn it on in your smartphone) you can use your home provider as if you were at home (calls to Thai numbers are international then!) whenever you’re in a WiFi.