Bharat Sanchar Nigam [BSNL] — not being able to make calls

I am not being able to make calls from my bsnl mobile. Every time i try to make a call, it says recharge immediately, though i have balance of over rs 100/.
I am having this bsnl sim for about a year and i did not face problems till about two weeks back.
When i tried to call, it was just quiet. After several attempts, it said, "recharge immediately". I recharged, i got confirmation of a balance of rs 75/, i called and the call got through.
But next day, the same prob;em with the same advice, "recharge". I tried to recharge online, could not succeed. I recharged from a shop and received confirmation of having a balance of over rs 100/
But when i tried to call this time, got the same advice "recharge"'.
I do not understand what the problem is and the bsnl helpline did not respond.