Batman: Return to Arkham - Arkham City

Pretty early in the game you will hear a phone ringing (and get a notification on the screen, combined with a distance indication to said phone). Go there and press to answer the call.
This will start the cold call killer side-quest and will net you this achievement.
The location of the first phone that you will hear ringing is completely random. They are scattered all over Arkahm City. Personally, I found my first ringing phone after the catwoman sequence in which you meet poison Ivy for the fist time. As soon as I came close to Mr. Freezes location I heard a phone ringing.

If you want to get more details on the whole side-quest I refer this guide right here:
http://guides.gamepressure.com/batmanarkhamcity/guide.asp?ID...

Hope this helps you guys!
Any improvements? Please let me know in the comments below!