This might be totally off-subject, but have they changed local dial tone providers lately? It sounds to me like your problem may be as simple as not receiving disconnect supervision on the CO lines.

We have see this happen on other brands of systems, usually during night hours where the voice mail or auto attendant answers all calls. Since disconnect supervision isn't being received, the voice mail ports don't ever release the call and you end up with all lines lit up. Maybe next time it happens, you should check to see if the voice mail ports are locked up at the same time.

We have seen this loss of disconnect supervision even happen with existing customers who don't change providers. The LEC just rearranges some equipment in their network and it just goes away on it's own.

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Ed
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How come there's always enough time to go back and fix it a second time?

Ed Vaughn, MBSWWYPBX