@ALl BSNL users or becoming one, please read my reply carefully as I have more experience with telcos than anyone else here.
1. BSNL has different case from pvt cos. Their validity is divided in 3 parts.
a) Active Validity b) Grace 1 Validity (7 days incoming) and c) Grace 2 Validity (165 days number won't be deactivated and can stay without recharge).
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Suppose you have an active plan i.e you recharged with 199_28 days plan today i.e Oct 11, 2025.
This pack will work till Nov 7 and this comes under Active Validity.
Once Outgoing expires from Nov 8 to Nov 14, IC SMS and calls will continue to work. This is Grace 1 validity.
Now from Nov 15, 2025, Grace 2 starts and for next 165 days which will end on April 28, 2026 after which your number may or may not disconnect as sometimes system moves sim to Grace 2E i.e Grace 2 Extended validity for another 90 days but it is little bit risky.
2. BSNL don't follow this 90 days Rs.20 deduction rule.
If you keep balance in BSNL, they deduct Rs.2 from balance after active validity expires and in some circles you are provided 1 day validity and in other circles 2 days validity. In most B and C category circles they provide 2 days validity.
How this works?
From example above: Your plan expired on Nov 7 and you have Rs.50 top up with Rs.39.50 in account as 50 topup gives 39.50. At midnight starting Nov 8 between 12:00 AM to 12:30 AM, they will deduct Rs.2 and provide you 1 day or 2 days depending upon how they charge in your circle.
So now you will have 37.50 in balance and OG/IC validity for Nov 8 and 9.
On Nov 10 they will again deduct Rs.2 and this will keep going until balance becomes zero. Here with 39.50 you will get 38 days validity if 2 days provided or 19 days validity if Rs.2/1 day provided.
After that this 1.50 leftover will be cleared to zero once Grace 2 again starts.
You can dial *123*1# in BSNL to know all the details but it will work only in 2G/3G network as USSD don't work in BSNL 4G currently.
Now comes the thing of base plan with BSNL.
BSNL keep a base plan and to know which base plan you have dial *123*3#.
If your base plan is Rs.75, never keep balance above Rs.74 once active validity is ending. The funda heere is if you keep balance above base pack on your number they will deduct it all and give you 30 days validity with some freebies. Keeping 74 will keep number active with Rs.2 auto deduction process which BSNL has. If you have recharged with say 100 top up then it gives 81 balance. Use it for SMS to premium number which costs Rs.3 and bring it down below 75.
Some numbers have Rs.106/107 as base plan so they can keep 100.
Now comes the until when you can top up in BSNL?
1. In active validity phase you can recharge between Rs.10 and above with any amount divisible by 10 like 20/30/50/100/150/220 and so on but you won't need more than 50/100 to keep balance low.
2. Once you enter Grace 1 i.e period of Incoming only which is Nov 8-Nov 14 in above example, you can recharge with minimum Rs.50 and above top-up only till Nov 14.
If you don't recharge from Nov 15, you can't recharge with top-up and will have to use a base Plan Voucher like 147 to bring number back to active validity.
In Grace 2 incoming calls and SMS stops completely but in C category circles it may continue.
Any queries? Shoot question here.