This issue comes on Zong prepaid sim without internet bundle. Everyday Rs. 20-30 balance reduced (rs. 4 each time). It's fairly common issue for those who don't use internet packages on Zong. Zong knows the issue very well, but they never resolve.
Balance even got deducted on old Nokia and qmobile that did not have gprs etc. I called Zong and told them and they were like ap mobile data use kar rahy hongy (on old Nokia too), wth.
Then switched to Telenor, same issue there except the deducted amount is Rs. ~5 per day.
Mobilink & Ufone are working perfect, did not try warid.
It's not settings issue, it's just fraud that earns them 10-20 crore everyday. They know the problem very well.