So I agree with you that's it's an ARG, but transmitting intelligence (or, more typically, instructions for agents) over unencrypted channels has been going on for years - the security of the channel itself is irrelevant if the data you're sending is only usable by the intended recipient.

This is how numbers stations work: you send alpha-numeric codes over shortwave, which assets in the field then decode using one time pads.

So generally speaking, transmitting intelligence over an unencrypted land-line wouldn't shock me in and of itself.

http://en.wikipedia.org/wiki/Alternate_reality_game (I didn't know either.)