yet-another-intercom-hack

Concerning the "ERROR: Unable to mount SD card : NOT_READY", I guess it's because Espruino tries to look for a connected SD card which may contain the module that's being loaded

Yes... but that only happens if the Web IDE hasn't uploaded the correct stuff.

Did you just paste code into the left-hand side? When you use modules you need to use the right-hand side, so the Web IDE can properly scan your code and load up the modules you're using.

If you want to use the left hand side, just put require("AT") in the right-hand side and upload, and then you can do whatever you want on the left.