People who want to keep secondary number active with minimum hassles, bsnl/ mtnl is THE BEST bet. Not only they provide least amounts like 35 rs/ 6 months for validity, its long term also so, no need to recharge every month.
And height of minimal cost is, if you dont recharge at all for that validity, your sim still works for incoming.
I had not recharged airtel number for almost 2 years. May be recharged only once in 3 years. So after this stupid monthly recharge rule came, i ported this number to bsnl. And havent recharged even 1 rs or 35 rs/ 6 month as per rules. But that sim is working for incoming traffic.
So, no worries at all. Just like before. Only issue is bsnl doesnt have good network like airtel. And i had kept that number with airtel only because they have good tie ups with networks abroad for international roaming. That i will miss now.
Hopefully jio improves wrt international roaming tie ups.
May be OP should add this info in OP so lot more people who are not aware of bsnl charges, benefit.