When I connect the DX121 to my wired intercom headset connector, I hear an echo through the headset on my BP200 beltpac. How can I turn this off?

When I connect the DX121 to my wired intercom headset connector, I hear an echo through the headset on my BP200 beltpac. How can I turn this off?
Some of the hardwired intercoms provide a "sidetone" for their headsets. If the manufacturer does not provide a sidetone adjustment for their equipment, turn down the HME BP200 beltpac sidetone until the echo is not heard.