This offer applies to Optus prepaid customers who use their Android phones lock screen as an advertising platform. Along with free data, the telecom company are also throwing in free call credit to sweeten the deal too. This has all become available due to a partnership with New Zealand based start-up called Postr.
Optus Xtra is a free Android app that gives users 1GB of free data or $2 of credit every 28 days, across a variety of Optus prepaid plans. Basically, in exchange for you downloading the app and allowing it to use your lock screen to advertise, they reward you with free data and calls.