I never worked in the related Depts. But, based on the usage experience n the way banks charge their customers, I expect the following :
Every trxn, be it balance enquiry or withdrawal or deposit or other ancillary service (like PIN change....), is charged by the relative network. Banks have got to pay. How much n at what periodicity may be different for different banks - based on volume n type.
If you see the IBA guidelines or the detailed TnCs of various Banks, after a certain number of FREE interactions, customers are charged thereafter for every trxn.
Like in any business, there might be discounts, seasonal offers, competition-induced schemes n reliefs.