Wait after all that he could have just called his number?!
The code to unlock the disc found in the wall "You are here" ... Is the note found in the box next to the tree. The disc has the phone number. So yes, He had to dig up the box.
I'm not so sure the code is this note. If you decipher it and read again the previous comic, it takes a whole new meaning ;)
Maybe I overthink this a bit. It could also be this note of course. Anyway, it's fun, and I admire the authors for finding this idea.