I saw something in another post a while back reporting a very similar problem on a Panasonic 336 PBX. I'll bet another beer that there is a corroded jack or connection for a station that's causing an intermittent short which results in pulses being sent. Since you mentioned that is is a small property, I'll guess that it is older and existing wiring was reused. I can't tell you how many small motels or B&B's around here have station wiring run outside with exposed "maintenance man" splices.

One question: Can't you program the station ports on that switch for DTMF only? That might stop the issue until the failing station cable or jack goes out completely, making it much easier to locate!

While my potential solution is somewhat simple, finding the culprit is going to be like finding a needle in a haystack. Good luck and please keep everyone here posted as to your progress and outcome.

Ed Vaughn, MBSWWYPBX