Update:

As many users have commented they are unable to use this service becoz The Airtel people came to know about free GPRS. So airtelfun.com & airtelmms.com are both firewall-ed now. You will get connected but every request is failing now. So no use of all the settings..

But don’t get dishearten as Rahul @ Devils Workshop comes to the rescue he has posted Free Airtel GPRS Trick is Back just check it out for the latest settings…

You need a PC or a Laptop and the required connectivity tools ,ie.,
Serial/USB cable OR Infrared Device OR Bluetooth dongle

1) Activate Airtel Live! ( It’s FREE so no probs)

2) Create TWO Airtel gprs data accounts (yep TWO) and select the
FIRST as the active profile.

3) Connect your mobile to the PC (or Laptop) and install the driver for
your mobile’s modem.

4) Create a new dial-up connection using the NEW CONNECTION
WIZARD as follows

Connecting Device : Your mobile’s modem
ISP Name : Airtel (or anything you like)
Phone Number : *99***2#
Username and Password : blank

5) Configure your browser and download manager to use the proxy
100.1.200.99 and port 8080.( My advice is to use Opera since you
can browse both wap and regular websites)

6) Connect to the dial-up account. You will be connected at 115.2
kbps (but remember, that is a bad joke).

7) Pick up your mobile and try to access any site. You will get “Access
Denied…”(except for Airtel Live!). IT DOES NOT MATTER.
Keep the mobile down.

8 ) On the PC ( or Laptop) open your browser, enter any address ,
press ENTER and…….WAIT

9) After a few seconds the page will start to load and you have the
WHOLE internet at your disposal.

Credits from:Thilak