Smartphone contracts can be expensive, especially if you want a deal with a sizeable data allowance. To help you save money, Tesco is offering an unusual deal -- put up with ads on your lock screen and in exchange, you'll get £3 off your monthly bill. The offer is being handled by Unlockd, a startup that's already partnered with Boost Mobile, a subsidiary of Sprint in the US, and Lebara, a network in Australia. You simply download the Android app, log in with your Tesco account and set some preferences. An ad will then be shown "every few times" you unlock your phone.