Also, there's a USB keyboard attached (the keypad) and I regularly read impossible characters from it when it's not even being touched: NULLs and low-numbered control characters. Note, this is not a serial port, these are making it all the way up the USB stack.
So I'm guessing there's some systemic grounding shit going wrong, or something. Any suggestions?
In other payphone news, it only took a month for one of the savages we call customers to completely destroy the rebuilt payphone handset. So that's great. I guess I have to re-build that from scratch now, since it's impossible to get the end caps off once you've screwed them on. Or, just toss the whole phone in the dumpster, because you people don't deserve to have nice things.