Hi, I have been through the same experience with National Australia Bank for 6 months, eventually I just gave up as I am of the opinion their technical networking personal are a bunch of wasted space.
They kept telling me no problem here in NAB via their CS support, must be your phone.
I have received OTP codes for years in Thailand, from NAB on the same True Thai mobile number and phone up until May 31 this year 2023, it all just stopped dead in its tracks.
I still, as of today, receive other SMS OTP codes from other institutions in Australia and all over the world, plus my friends back home in AUS send me SMS msgs regularly.
After great expense on international calls to and wasted time with NAB CS support for months and the fact I proved to them, it fails on both my DTAC and TRUE services on different model iPhones and Androids, I called it a day!!!!
Both True and DTAC told me the two numbers NAB send OTP codes on were not blocked, as for CAT Thailand, I am not sure about them as they are the primary infrastructure provider for Mobile carriers here.
The SMS international gateway networking system is quite complex, with often many intervening SMS servers that cater for variations in the different protocol conversions used in the various countries
The dummies in NAB networking operations are clueless to understand how the SMS end-to-end process works for international msg transfers,
Some servers do a function called store and forward, where the NAB originating gateway server provider gets a response that their SMS OTP has been delivered, where if fact it could be blocked further down the networking services due to security concerns of all the spam floating around the international networks.
The other issues is; SMS is a connectionless network transmission. This means that there is no established connection between the sender and receiver before the message is sent. The message is simply sent out into the network and is routed to the recipient's mobile phone.
All those knuckleheads at NAB Networks Operations have to do is contact their technical SMS outgoing gateway provider in Australia and have them engage their counterparts in Thailand CAT, True or DTAC and run an SMS network trace between NAB AUS and TRUE or DTAC Thailand SMS servers.
Would find the issue in a flash............but they are neither not interested or lack the skills to do so.
So in summary I had to divert my OTP mobile code to my son's phone in Australia, I's a real PITA, but it gets me over NAB's incompetence.
Sorry for the detail discussion, but it was so frustrating dealing with such poor technical support in NAB AUS.
BTW, I am definitely not an expert in this SMS international delivery area, but just use a common sense approach to understand the problem.