Be warned if these are lines coming off of an ATA of some kind (SIP, Cable, or other proprietary device) that some of these do not return a disconnect signal at all, or it is so short the Mitel cannot get it. I have run into this before with some of the local cable companies older ATA's. You can usually put a butt set on the line and "hear" the disconnect, it is typically a disconnecting of battery for a moment a few seconds after the remote end disconnects.