Airtel and Vodafone have official rights to sell iPhone mobile devices in India. Last year, they have sold many iPhone devices which were allowed to use with only one carrier i.e. Airtel or Vodafone.
Unlocking of iPhone is not allowed by Apple, but Airtel now allows you to unlock the cell phones. This means that you can use it with any carrier after buying from Airtel. Apple’s website displays the authorized unlocking of iPhone devices with Airtel carrier. Airtel has not confirmed this facility, but we can expect an announcement soon.
Here’s the screenshot of authorized unlocking from Apple’s website.
You can check the link here – http://support.apple.com/kb/HT1937