Though, there were already online services for paying postpaid bills, but the ability to recharge prepaid mobiles online is an innovative offering by BSNL.
For recharging your mobiles just head over to this page, and create an user id. Once you have access to login to BSNL portal, you can recharge your mobiles online at any point of time.
BSNL customer can recharge Prepaid mobiles online by logging into BSNL Portal
Mobiles can be recharged either with recharge vouchers or top up cards
Customer needs to use net banking a/c or credit cards to pay for the recharge
Customer will receive an SMS confirmation once the recharge has been done.
Link: BSNL Online Recharge