Follow the below steps to get unlimited internet on your BSNL:
1. First of all you need to recharge you sim with a small BBM pack:
I recommend Rs.97 plan but confirm it for your state
2. Go to your PC's network settings.
3. Right click your BSNL connection(make sure it is not connected)
4. Click on networking >> Internet protocol version 4 >> Properties
5. Now set the Following DNS servers in the DNS boxes:
Primary : 8.8.8.8
Secondary : 8.8.4.4
6. in your Phone set the APN to blackberry.net
7. Connect the internet through the created connection on pc
Enjoy!