A lot of banks use SMS OTP.  Some Kiwi wants to receive OTP while roaming, but don't want to turn on data roaming with NZ SIM and prefers to use local SIM while abroad.

SMS are currently transport with 3G signalling channel so you still get SMS delivered while abroad even though you don't do data roaming with NZ SIM while abroad (of course dual-SIM handset is used).   If my understanding is correct with VoLTE SMS gets delivered over 4G LTE data, not signalling channel.

With VoLTE being mainstream and 3G network shutdown in increasing countries.  How does SMS get delivered while roaming abroad in a country with no 3G network (eg. in Australia, Singapore) if you use local overseas SIM for call and data while abroad and have NZ SIM roaming but not used for data or calls?  Do you still get NZ SMS delivered?  How do you get charged for receiving NZ SMS?  Do you need to quickly switch to NZ SIM data while anticipating SMS reception?  Receiving NZ SMS over VoLTE while roaming while using local SIM for LTE data is a bit of a mystery to me.