I had one of these! You had to load the terminal software from the included cassette tape, pick up the telephone, dial the digits (literally, we didn't have TouchTone), wait for the Bell 103 handshake tones, then quickly unplug the handset and plug the cable into the modem. Set the handset aside so you don't accidentally hang up.
Then enjoy BBSing on the 22x23 display! Good times.
Then enjoy BBSing on the 22x23 display! Good times.