GN 9120 beeps constantly

Are you using the GN1000 Handset Lifter? And if so, are you playing music/radio through the speaker of your telephone? If this is the case the Lifter will be picking up the sound of the music coming from the telephone and interpreting it as an incoming call, hence sending a signal to the earpiece in the form of beeps. Just stop the music coming from the telephone and the beeps should stop.
If you are not playing music through the telephone, you may need to turn down the sensitivity adjustment on the back of the Lifter. Just turn the knob anti-clockwise a little bit and see if that eliminates the beeping. If the sensitivity adjustment is too high it will pick up every little noise that goes on around your desk and interpet them as the phone ringing.

If you are not using the GN1000 Handset Lifter, then the beeping may be the result of a sticking/stuck volume button on the earpiece. If you adjust the volume on the GN9120 it gives a beep when you reach the maximum or minimum volume. If a button is stuck down it will result in a constant beeping. A stuck button is generally not a user servicable problem, however a little bit of tinkering with the button may get it unstuck.