The names on the pcb don't get me anything useful from Google, but Intellon, the name on a couple of chips, was apparently a maker of powerline networking hardware. So presumably this device was used to network some device, connected to the modular jack, to other devices, using the building's power lines.
Edit: okay, I did find this in a guy's LinkedIn resume:
"CEO/Owner
Compuline, Inc
1991–2005
(14 years)
Managed development, marketing and sales of patented, 26-claim Coin Tracker automated collection systems, including 4 unique wireless networks: 1) Infrared; 2) AC power-line; 3) RS422 wired; 4) RF. Clients included Sega/GameWorks, Disneyland, Aristocrat, Chuck E. Cheese, Capcom and others."
So it was probably involved in networking games to a central computer over power lines for accounting purposes.