retrocomputing

It was a Radio Shack 300 baud modem. A little googling seems to indicate is would likely have been a Tandy DCM-3 “Direct Connect” (as opposed to acoustic coupler) modem.

It was in-line between the wall and a phone so you would pick up the phone, dial the number, head the modem tone, press a red button on the top of the modem and hang up the phone.