This means the 9120 needs be re-synced. This is done in a similar way to the Plantronics CS Wireless models, except it is a bit more difficult because of the location of the battery.
1) Turn the headset OFF with the headset out of the base.
2) Unplug the AC/DC power cord from the bottom of the base unit.
3) Return the headset to the base, and then plug the power cord back in.
4) Wait 5 seconds for the lights on the base to stop flashing, then take the headset off the charger and test it.
5) If it does not work, then allow it to charge for 1 1/2 hours and try to use it again.
If the beeping continues, then perform a battery reset following the instructions below:
1) Remove the headband/earloop.
2) In the headset receiver, where the battery is located, press down on the plastic tab securing the battery and pull the battery out. Note: If the battery plate has screws securing it, then it requires an eyeglass-sized screwdriver to remove the two screws.
3) After waiting for one minute, insert the battery into the battery compartment. Note: Make sure that the battery is placed correctly in the headset with the + of the battery corresponding to the + indicated in the battery slot of the headset.
4) Now test the headset.