Bharti Airtel refuses to refund the amount for the IR services which did not work

I had purchased an international roaming plan from airtel for my mother on 17th Feb., 2018, who was traveling to the US. On reaching US, my mother’s SIM card did not get registered with the available network. I called up Airtel customer support to register the complaint. After not getting any satisfactory response over phone, I posted my complaint over an e-mail. The complaint finally got registered with reference number CCNOR3022367448000.
The e-mail exchanges continued for about a month. The customer care kept on promising to resolve the issue soon, but the same did not get resolved.
Now, when the validity of the one month international roaming pack has ended, I am demanding a refund of the pack purchase amount (INR 3997) from Airtel. The e-mail support has stopped responding, the support over phone refuses to refund the amount, and the support over twitter completely deny any problems with Airtel’s services.
Looking forward for your kind help.
Thanks.