Airtel — refund of advance rental after discontinuing the service

Address: 560115

I paid a 6 month advance rental of rs 6903/- for my airtel broadband (User id in september 2018 and discontinued the service in november 2018.
A refund has to be made for 3 months for which the service has not been used. December 2018 - february 2019
Airtel has not refunded the balance amount for 3 months even after repeated calls to their customer care center.