The HC-1 Handi-Coupler provides a straightforward way to interface external audio equipment, such as a mixer or recorder, with almost any telephone that uses modular connectors for its handset. Because it connects in series with the handset and not directly to the telephone line, it can be used with a wide variety of phone systems, including single or multi-line, modern electronic, or older key systems, without requiring a direct connection to the phone line itself.The unit is a passive device, meaning it does not require any external power to operate. A simple on-off button on the front panel toggles the connection between the normal handset use and the external audio input/output.