Last month it was 28 days on Amazon as well.
After i put the Port Out request, they took the case seriously and called me to apologize for the issue.
The incorrect validity was not because of 24 or 28 days, but because they started the new pack on the day of recharge instead of stacking it on the existing unlimited pack.
For context, my validity was supposed to end on 20 Nov, and i recharged on 17 Nov. (future recharge..which we often do..)
They started the new pack on 17th only.
Reason? : The old and new recharge were of different denominations. How dumb.
Thus even though the system had posted 28 days of validity, it was effectively ~24-25 days.
Today the new associate cleared it out, apologized, and added some balance for the company's mistake.
They sent me these TnC:
1. For Unlimited Combo plans, customer can now do multiple recharges at the same time and the validity will be accumulated for all the recharges made.
2. Maximum accumulated validity can be of 2 Years(730 Days)
3. Validity will be accumulated only for same denomination recharge, having same Offer ID.