I only turn it off at phone level and not the app. If app level is turned off, I won’t be able to get mobile signal and won’t be able to receive SMS.
Just to share that when I was in China last week, SingTel decided to auto subscribe to the Asia Roaming plan via ReadyRoam, even though my data roaming was disabled on my iPhone.
By right this should not happen. What you do is the recommended way to use Singtel SIM Only, if you want to receive free incoming roaming SMS.
We have two Singtel SIM Only plans and we never got S$18 roaming charges with the four recent visits to China with (my wife: three times, me: one time). Same that we do not use Singtel for roaming data but using eight or other means like travel eSIM. In fact, I have never got roaming charges this way for so many years.
Maybe you accidently enabled data on the phone. Sometimes it becomes tricky with some settings on the phone, like dual SIM Android phone.
Example from my Poco X7 Pro.
1) Main SIM -- Singtel SIM Only, not for roaming, data disabled, roaming diasbled on the phone (MySingtel app still allow ReadyRoam).
2) Secondary SIM -- say eight mobile for roaming usage, data eneabled, roaming enabled. Used as the primary during roaming for data and voice.
In the phone, there is a setting to "use mobile data during VoLTE calls on the secondary SIM" which you may enable in Singapore. But I tend to think it may cause issues during roaming.