Have replaced or isolated all packs and carrier. Have tested or removed all extension wiring. Have installed an APC? battery backup which indicates no issues through it's software. The analog sets are polycom units with headsets. They have lights when in use, but I've removed half of them in case there was too great a current drain when in use. It seems the only other area might be power surge on one or more c/o lines or a problem with one of the 5 18D sets. Any more ideas?
