This achievement is basically story-related. Juliet receives phone calls randomly throughout the game, there are 21 messages and the last of them will come during stage 6, so you will get the achievement right before the credits. You don't have to listen to the messages unless you want to, it's not required to get the achievement. If you do want to hear the messages, then hit pause and go to Juliet's room/stash and the phone messages are in one of the tabs. This can be done any difficulty level.