A handset device and an interface unit used to transmit at least one of a command signal and an audio signal to a hearing device by way of a wireless connection are disclosed. The handset device has an audio output and is configured to transmit a power signal configured to power the interface unit to the interface unit by way of a wired connection connecting the audio output of the handset device with an audio input of the interface unit. The handset device is also configured to transmit the command signal and/or the audio signal to the interface unit by way of the wired connection. In certain embodiments, the audio signal and at least one of the command signal and the power signal have pre-assigned and non-overlapping frequency ranges.