Please follow the procedure below:

1. Check the audio link to the base station by a constant blue LED activated on the front of the BW 900 base station.
2. Check the Microphone Mute function is not activated by not having a blinking red LED activated on the front of the base station. 
3. Check that the microphone sensitivity adjustment on the side of the base station behind the steel lid and set set to the appropriate setting for the telephone in question.
4. Check that the headset microphone is positioned correctly near your mouth.
5. In case of "humming", your phone might be very sensitive to radio frequency signals. Move the base station and other radio activated mobile units farther away from your office phone.