Sometime the ops ppl quite kuku. Why press 1 to confirm, 2 to change and 3 to cancel. **!
Just preset 1 to change, 2 to cancel. Never press anything means coming.
SMS is not 100% confirm/guarantee delivery (OR even guarantee timely delivery).
There is possibility that:
Although a SMS can be sent out -- but the receipent may not actually receive it at all, OR the receipent may receive the SMS a LONG time after it is sent out.
(got a few / various reasons but of course such cases don't occur often)
So (usually) SMS OTP has time limit & (should) give you the option to request resend of SMS OTP.
Then how to know if the receipent has received the SMS that is sent out?
Ans:
Solicit a reply SMS.
If the recepient gives you a reply SMS -- then that indicates the recepient has received your SMS.
You may ask why not just send notification via mobile app & solicit a reply via the mobile app?
Ans:
SMS can be sent & received --
WITHOUT the need for Internet access at all.
You can send a SMS to anyone & receive a SMS reply -- regardless whether the recepient's phone has WiFi or has mobile data access or not.
(In fact, you can send or receive SMS -- even if your phone is not a smartphone.)
If use mobile app to communicate; 100% means Internet access is a requirement + (SMS) recepient MUST have smartphone that cannot be too old -- since many/most mobile apps don't support Android or iOS version that is more than 4/5 yrs ago.
Of course nowadays, not every entity/party you engaged with will communicate w/ you via SMS -- so it's up to you to decide whether having the ability to reply / send out SMS is worth to have (when overseas).