Just a minor correction: Not every Axis credit card gives 25% discount on Airtel payments. It's only the Airtel Axis co-branded credit card that gives the cashback. This card has very less approval rates these days due to high number of applications and are only given to select customers now. I have even shared my on-boarding (approval) experience for this card here:
Airtel Axis Bank Credit Card
It has other benefits too such as 10% cashback on electricity, water, LPG gas etc. bill payments.
I have been using this credit card from last year and it's totally worth the hype. I don't recharge Airtel wifi since I have no connection yet and getting good amount of cashbacks with rest of the eligible payments. With an Airtel wifi connection, the cashback will be further increased/doubled.