BSNL is the way to go for safekeeping secondary numbers at cheapest cost. Don't expect anything spectacular from 3G(if it works) but voice works fine in BSNL but they have those blackout days
My primary number is BSNL for the last 7 years. But after Jio, it's in my phone for receiving only. Outgoing is very rare. Didn't recharge it in last 3 years, still balance in 4 digits.
Edit: Just checked, it's actually ₹862.4446