Airtel — Wrong recharge and not being refunded

I have mistakenly recharged another number which belongs to my grandmother instead of my number through Google pay. I have contacted both google pay and airtel and both of them are denying to help me. Google pay gave me reason telling that the amount have already reached airtel and they are not able to do anything.
Airtel is denying my refund by stating that I have used 3rd party app to recharge.
Here are the details:
Wrong No : [protected]
Transaction id:[protected]
Recharge amount: 399
Correct No: [protected]
I have seen many people telling that they have got refund but why is Airtel denying my refund