at least as well as an untrained illiterate 3-year-old :)
Here is a way to overcome the illiteracy issue for communication over a text-only channel: ASCII art. Took my lazy and pretty average mind all of 10 seconds to come up with it. And to the AI in question all humans are basically illiterate 3-year-olds. We won't know what hit us. Also, I cannot resist bringing up this piece of fictional evidence.
I hadn't considered ASCII art!
a) Blind people, then.
b) The idea that an AI, with no clue who is on the other end of the line, and no feedback from the 3-year-old touching the keyboard, would be able to correctly extrapolate what it's dealing with AND produce the exact-correct stimulus with 100% accuracy... strikes me as straining all plausibility. Fundamentally the AI needs some information to get feedback, since there don't seem to be any universal-to-all-humans hacks out there. But if you have built an AI that can do all that, then I can't help but thinking that putting it in a box is pointless anyway...
Update 2013-09-05.
I have since played two more AI box experiments after this one, winning both.
Update 2013-12-30:
I have lost two more AI box experiments, and won two more. Current Record is 3 Wins, 3 Losses.