I had the similar problem with one that had a system phone located several hundred feet away in a non-conditioned barn. When it was humid the system programming would get all messed up. A POT worked fine. Pana tech support and this site offered suggestions but none worked. Apparently water condenses on the pc board assembly in the 7730 phone and causes the problem. I eventually had to swap it out for 816 KSU, same phone, no problem. I later reused the same 624 at another site and had the same problems. A system phone at about 200 ft in workshop that was sometimes conditioned, sometimes not caused same problem. That 624 is yours if you pay shipping. I won't use it.
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