Batman Arkham City

This is one of the easier achievements in the game. It can be done before and after the game's been completed. Whenever you are outside you may hear a phone ringing. There are several phones in the city and the game will tell you to go into detective mode (LB). now if you look around, you will spot some sound waves. go to them and you will find the phone. Press A to start a side mission and even before you start the achievement will unlock.

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You can get this when you are looking for Freeze. After you take care of the bad guys outside, before you go in there will be a phone ringing just outside. Go up to the phone and answer it, listen to the call and achievment unlocked.