You make an interesting defense of Citibank. First of all, I am being fair to them. I followed their advice and updated my phone number to the USA one. To say I made the mistake of having the wrong mobile phone supplier is crazy. I should not have to worry about that just because of Citibank.Yes Citibank are annoying, but to be fair to them, your biggest mistake here was having the wrong mobile phone supplier.
I am not sure what you are proposing by using the Aldi pre-paid when overseas. I have good reason to have a local number when overseas for any length of time (e.g. try using Uber without a local number when drivers try to call you). So international roaming generally doesn't work for me. Or do you mean carry a second phone just in case Citibank try to contact me (no thanks)?
As for the Digipass, yes that is a good tip, but given I didn't know about it and it is not publicised, it is hardly my mistake that I didn't ask for it. Regardless, as I said in my original post, they knocked back transactions sporadically and only once tried to contact me at the time of purchase, so I am not sure it would have made any difference aside from that one transaction.