>In 1982 many companies had modem connections to the LAN line. We began receiving telephone calls that sounded like a modem trying to make connection.

These were at least real modems trying to connect.

The admin of that bulletin board in Belgrade actually learned to imitate the sound of the modem. He'd call from a bar or anywhere, beep with his mouth, and check the response on the other side. The modems in the office would get stuck on a whim, and he used this just to make sure they still responded.