Billing system of BSNL depends mainly on 3 departments. Your local exchange (also called Internal), IT Cell and AOTR (Accounts Office Telecom Revenue).After every two months (as billing is mostly bimonthly, in some cases billing is done monthly or fortnightly) the local exchange run a software which produce a list of phone number and respective opening meter reading and closing meter reading. The software also automatically writes all these data to a CD.Then the local exchange or Internel department sends that CD to the IT Cell. The IT Cell inserts the CD to the Cd Drive, run a software which prints the bills in the format you get every two months from BSNL. IT Cell also despatches the bill to the corresponding addreses. The job of IT Cell is done.Now comes AOTR. Assume the last bill payment date is 30th June. On 1st July, the AOTR will run a software to get the list of defaulters. The software will make a list of defaulters and write the datas to a CD. AOTR then sends this new CD back to the local exchange(Internal).The Internal staffs will run a software after inserting the CD sent by AOTR and the software will automatically make the defaulters phoneline oneway.This is the system adopted by BSNL for billing.In this forum I have seen some posts which says that BSNL billing system is centralised. It is virtually impossible to implement a centralised billing system.Different circles of BSNL uses different software for billing. Now the standard BSNL billing software is DATA-SOFT. It is used by BSNL AP circle. Before DATA-SOFT the standard software for billing was TRICHURE. BSNL West Bengal circle uses TRICHURE. BSNL Calcutta circle uses neither of this two, but their own.