System installed with latest software and firmware in both. I have also replaced both units. Line 1,3 ringand 2,4 are respective rollover lines that go to vmail with no ring. The have a master message and 2 mailboxes from a custom service menu. Once a week roughly line 1
is locked up and if you pickup l1 as a conference you can hear the vmail and the custom service greeting playing. I have changed the cpc monitored the line and cant find anything. The incoming are pots and the dsl is piggybacked on
L 5 not on the system. ATT says not a fault of their lines. Sorry for such a long first post. Any insight would be great.

Have you verified that you are receiving the CPC disconnect?
I don't know how you can "conference" in on an active line unless your referring to "executive override"
Either way, do that again and then open the line to see if the calling party never hung up.
If it drops as soon as you open the line then you know your system's OK.
It may be a machine of some sort that stays active as long as it hears voice.
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MrG

Thanks for the quick response.Yes Executive overide is enabled. When you press the line 1 which is usually the line that has frozen there is of course no other party and the system is then freed again. We have placed outboard cid boxes on 2 of the incoming lines 1,3
to try to understand type of call,cel,machine,pbx based or such that may be causing this as well. Unfortunately no rhyme or reason has shown up. As it seems to have happened from all types.