Could be possible to play modem dialup over the internet, but would need some additional (moderish) equipment.

Hardware solutions along the lines of the Lantronix or MOXA ethernet modems (basically hayes modem emulator, serial to ethernet converter) could be used, but I suppose both parties would need the same brand converter to make it all work. Alternatively, perhaps TCPSER has this functionality, then just a reasonably common old windows box could be used as the go between on both ends.