SingTel Prepaid Card balance deduction even when not in use.

Hi. I had 2 sim cards in my phone. Sim 1 is my Circle.Life card which I used primary for data, call and sms. Sim 2 is my Singtel Prepaid card which I keep in my phone for receiving OTP.

Since this year, I realised that the balance in my Singtel prepaid card has been reducing by 6 to 7 cents daily. Mobile data for this Singtel Prepaid card has all the while been turned off. I only use it for receiving sms and I do not use it for data, call or sending sms.

I tried calling Singtel support and the CSO said their system reflect data usage from my Singtel prepaid card and they are unable to tell me why there is data usage. I can only say Singtel support is not helpful towards prepaid customers.

So I have dialed *360 to turn off data for my Singtel prepaid card and there has been no 6 to 7 cents deduct since.

Now my puzzling questions are:
1) is it my phone problem? I have been using the same phone last year and this year but this problem started surfacing this year.
2) is it some shady tricky from Singtel to make prepaid card user topup their card more frequently?
3) is it my android one OS problem that Google is sending out data in the background?
4) does this problem happen to other telco prepaid card user such as Starhub and M1?