Which honestly is another design flaw. Not required then don't show it, or make it work correctly. And that includes being able to empty the field, not answering.
Do you have a google (gmail) account? Then just use Google Voice. You get a real phone number that your US based contacts can call, you'll receive their call on your mobile. And you can call them from the app (for free). Note; you might need a US-phone number to receive an 'activation code' for google voice.
That only works if you are still registered with municipality in the NL. Otherwise you'll find out mate. Just read your banks website, no need to believe me.
Situation for NL is same as mentioned by OP, when no longer having an address in the country. In case you remain in EU you can keep some bank account at extra costs but i.e. dont have credit cards anymore. If you leave EU a nd do not have an address in the country they close the accounts.
While I feel the OP should consult professional advisor for his situation. And I am completely unaware of AU rules. And I am not a tax expert. But having said this, it was my understanding that if Thailand applies a higher tax rate over the relevant income than what has already paid in Aust, then Thailand could choose to tax that "gap".
(i.e. ignoring all else and just a random number it could look like 30% relevant tax rate in TH and 15% relevant tax rate in AU --> this leaves a 30-15 = 15% tax gap to get to the highest possible tax rate; if TH would tax you on this it would not be double taxation but applying local rules).