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SV 8100 sporadically not disconnecting when the call ends. This happens several times a month.
Voltage on FiOS Hunt group line(s) is working properly. Internally the main hunt group number is busy, externally the main number rings unanswered. The full time line recorder is recording attempted unanswered calls. The calls are coming in, The PBX is not available to answer the call. Installer is pointing finger at Verizon Supervisory Disconnect.
Thank you in advance for advise you can offer.

Installer is correct, regular POTS line rarely (if ever) have this issue. If that line recorder is on the line it could also be causing issues depending it that tap is truly passive. The only way to get around this "issue" that I have found is having the PBX auto attendant require someone to dial something to leave a message then timeout to hang up the call. The other way if you are REALLY REALLY busy is to be on SIP or PRI service. Ever since the industry has switched to these ATA boxes it has been a very serious issue that is not getting resolved and a LOT of finger pointing when the problem is obvious to techs in the field. I would also verify you do NOT have call waiting or 3 way calling on those lines either or any other feature other than hunting.

Coral Tech: Thanks for replying
I am checking on the line features as you recommended.
How can I check if the tap is truly passive?
The FiOS lines are analog, Does this make them POTS lines?
We were having noise on some of our copper lines, switching to FiOS analog was the
solution to that problem. Ever since the switch we have been having sporadic disconnect problems and
it is not always the main hunt number.

I am assuming you are using a call recorder on the lines correct? They can sometimes cause issues. Pull the recorder for a few days (if you can) and see if it goes away. Frankly, from my experience it's ALWAYS the ATA box. It's everything from timing to voltage. The big thing I have seen on busy systems is that for some reason they don't seem to keep up the voltages. Check the punch downs.