You need to insure (for sure) that the disconnect signal is being received from the far end. In the DX80 manual look up "CO Line Loop Supervision (Talk/Abandon)" Make sure you have each CO line set to yes on Call Abandon. And then try adjusting to Talk Abandon Timing to match the disconnect signal from the CO. Still having problems then have customer put in writing to carrier requesting new circuits. I had a DX80 with voicmail that would not disconnect on lines 3 & 4. After fighting with Bell and twice changing out the entire equipment with the Bell tech watching and moving lines up to 1 & 2 with problem following, I had to go up the ladder at SBC and get two new circuits. Once they put in the new circuits everything worked fine. The weird part was that I could hear the disconnect signal coming from the CO and adjusted the timing and still had the problem. Customer wouldn't pay me until resolved and I was about to give up a $6000 sale just because of this issue.