It should be quite easy. Chances are, there's no insulation in that inside wall to get in the way. I would pull out the existing outlet and look into the hole with a light to verify there's no electrical wires or plumbing before you cut from the other side.
Then you're all set. Simply copy the existing setup on the other side, connecting the phone wires from the old jack to the same colors on the new one. (I'm assuming you're adding an extension, not a new, additional phone line.)

Thanks for the advice. I didn't think it'd be that hard. I know there's no insulation in the wall.

Mark

One thing you might need to consider is the depth of the box. I use electrical boxes for my phone lines, and the standard boxes are 2-1/2 to 3 inches deep, so you can't put them back to back in a 2x4 wall. When I've had to use the same position on a shared wall for wiring I used two 2" boxes so that they fit, but I had to be careful of box-fill code. Since you're using phone wires you don't need to worry about that, so you could easily use 2" (or even shallower) boxes. Home Depot carries a variety of boxes, so you should have no problems.

As an aside, I use the boxes that compress to the wall and don't need to be affixed to a stud. It consists of a box and two tabs that slide in either side of the box, pressing against the inside of the wall. The tabs are then folded over the wall of the box and are pressed up against the inside of the box. It doesn't take long to get used to these, and they are quite strong.

Good luck,

Mark

Since you're talking about communications which is Low Voltage, you don't even need the conventional electrical box.

You can use a low volage mounting bracket such as this, go to the webpage below
http://www.aifittings.com/l_2.htm