Ultrasn0w unlock any firmware’s downwards from 3.0. You can use Airtel, BSNL or any other GSM network in India. I put some steps how you can unlock you iPhone 3G
1. First you download iPhone 3G 3.0 OS from the apple
2. Secondly update iPhone 3G without any worries, jailbreak via Pwnage 3.0 tool or via redsn0w tool .. some links from where you can download these tools
Pwnage 3.0 for Mac OS:
Redsn0w for Windows OS:
3. After jailbreak you can add Ultrasn0w from Cydia.
Add & Enter repo666.ultrasn0w.com URL as a repository URL and update, then install Ultrasn0w from the list of software’s ..
4. After installing Ultrasn0w, reboot you iPhone 3G.. its ready for different sim. Test it !
Note : This tool does not work for iPhone 3GS.