Change line ports. Perhaps that particular port is bad--i.e. "sensitive" due to past over-voltage. See if the problem follows the port or the line. Do you have lightning protection on your lines?? Do you have the system on a UPS?? Phone systems are on 24-7 and can get a bit goofy after a while because of all the surges, over-voltage, spikes, brownouts, blackouts, etc that they receive every year. It would not be an extreme statement to say that you have had probably 20-30 of these incidences a year. New systems really require all of this protection right off the git go. The older systems seem more robust but after 19 years, I'm sure that those incidences have taken their toll. Good Luck

Rockford VoIP