Airtel — Broadband disconnection request not handled

Website:

www.airtel.in

Hi, I have raised disconnection request. customer care suggested me to keep the modem for 3 months and there will be no charges, we will disconnect after 3 months. this is our regular procedure. i got a bill from airtel after 1 month as safe custody charges. i called customer care they waived of charges and said your connection will be disconnected now and we will pay the security deposit what you paid. Now after 3 months they sent a bill for full 3 months broad band charge and asking me to clear other wise threatening me for legal action.
how to handle this situation?