Post Paid Mobile Bill: Extra Charges for GPRS -Rs 28,000/-

Dear Sir I have been using a post paid service from a reputed telecom provider. On Aug-24, I have visited Japan and therefore was out of coverage. On Aug-25, I have given a request to change my plan (from 666 to 75) and requested for information on International Roaming to receive banking related messages only. I got a confirmation from service provider that plan will be changed in 48 hrs from 666 to 75 (basic post paid service). Plus they said Basic International Roaming will be activated for Rs 149 in 48 hrs. On reply to this I have clearly told No Internet is required in the International Roaming. However on Aug-25 it self, International Roaming was activated and I got a notice for High Internet Usage due to GPRS of Rs. 24,000/- in the night. I have not taken my mobile out of my room and since from India GPRS and Internet was active, here also the data was in active mode. I never got a confirmation, that International Roaming is activated and I have requested for No Internet. Initially on Aug-26, when I reported the matter, the representative said let us discuss once the bill will be generated. Without my notice they have deactivated my sim from Aug-26 onwards. Now they are demanding to Pay around 28,000/- for this including Tax. I have cleared my regular payments of July-Aug bill cycle except this additional GPRS usage fee. Though I have made several email and telephone requests to the Nodal and Apellate authority, they are demanding for making the payment. Now they are putting late payment fees on the due amount, and demanding urgent payment. Please advice what can be done in such scenario.