The best validity extension plan in BSNL is 'PLAN 153'. It costs ₹153 and the validity is for 365 days. It comes with ₹103 talk time with unlimited validity.
The best part of the plan is its tariff.
Get all BSNL to BSNL calls at ₹.15/min and BSNL to other network calls at ₹.40/min for the whole 365 days.
SMS cost is ₹.15/sms
It is bundled with 350 MB data with a validity of 30 days.
It's available in Kerala Circle and don't know whether it's circle specific.