Airtel just offers the free 500mb data usage to iPhone users who bought iPhones from them for 1 year. That does not mean you cant use their regular Mobile office service with iPhone which is not locked to airtel.
It has been long time ( sept, oct 2009) since I used iPhone, but still here goes.
You just edit APN settings under Cellular Data.
If its one of the phone which is bought from US and does not allow you to edit APN settings( I dont know if Airtel locked phone allows you to edit APN or not either ), there were 2/3 methods back then to do the same. One was OpenSSH method, other was safari method and iirc there was APN changing app in Cydia( at that time it was only for 2G and 3G iphone and not for 3GS, other two methods used to work with 3GS ).
Just google for it, you should fine some guide somewhere.