After spending my first 30 minutes on this map wondering where the heck that damn phone was I thought that the above solution was just a bit too vague.
First, you're going to need 2,750 points just to get to the room with the phone.
After a few rounds, actual number depends on how many players you're with, you should have at least 2,000 points. Spend the first 1,000 on clearing the debris blocking the stairs.
Right in front of the stairs you'll see a door labeled "Comm Room" that'll cost you another 1,000 points to open.
After opening the door, run through it to the hut and the last 750 points will be spent on opening the door. From here simply run to the right through the hole in the wall.
Once in this room face left and you should see a table with a black rotary phone on it. You'll need to hold "X" a couple of times to get it to dial. After the brief dialogue the achievement will pop for you.