You may be experiencing a feedback issue between the your headset microphone and the SCI box.
An easy way to test this is to unplug the headset from the suspect SCI box and then plug a headset from a different station into the suspect SCI box. If you find that this technique works, you will need to have your headset replaced (Please see "How do I have my headsets replaced?" for information on replacment procedures). If you do not have success and the problem persists, please contact SoundTree Technical Support at 1-800-963-8733 x6688.