1) Load up the zombie map "Shi Nu Numa".
2) Go downstairs and open the door entitled "Comm".
3) Fight through the swamp until you see a shack.
4) Open the shack door and enter.
5) Go through the door to the right of the perk machine.
6) Turn left as you walk through the door.
7) You will see a table with a black phone and a radio on it.
8) Go to the phone and hold "X" a few times while looking at it.
9) You will hear the phone dial and the phone will ring.
10) The achievement will pop up bagging you 10G.

Here is a video tutorial: