Attendant Call Waiting Going Off

[SFSRJSTW]

Twice now in the past week, our attendance console has had the call waiting light and tone going off even though the console is active, position is not marked as busy, and calls can be recieved and made normally from the console. There is only one console. So far the only thing I've been able to do to clear it is busy and release the port. Does any one else have any ideas of what might cause this, or how I can find the supposed waiting calls?

Thanks!