I did indeed post about this on reddit! The little door is missing from my machine. Inside, there's a concave metal plate filling the hole where a phone jack would be. I've opened the case up and the modem card physically isn't present.

I've got a LocalTalk network that I can use with this machine at home (as soon as I get it out of the boxes from moving, at least), but the modem would be nice to have too.