I have a BCM450 with three phones in a "Ring Only" hunt group. After the caller ends the call, the "intercom" button stays lit and that phone stops receiving calls. If you go off hook and press the intercom button, the message in the display is "line in use". The only way to clear this is to change that extension in the Hunt Group to "Appear & Ring" and switch back to "Ring Only". Any suggestions as to what is causing this?