I will be you that your phone system is a KX-T or KX-TD system using hybrid ports. These ports are unbelievably sensitive to any kind of exposure to foreign currents, like those from even mild lightning storms. There's nothing wrong with the cable; the added resistance from the length of the cable amplifies the problem that might not be as noticeable right at the KSU. The ports might sound clean there, but the minute you add the resistance and capacitance of a cable run, the problem can be heard.

What has happened is that the port has been blown (well, not completely), but blown enough to where even a few dozen feet of cable result in cable pair imbalance. It doesn't matter if the cable is inside or underground.

You might be able to move to other port(s) to clear the problem. There really isn't any type of protector that will prevent this from happening again. Panasonic's hybrid ports put out enough ringing voltage where digital station protector modules can't be used. Regular CO modules (230 volt) allows too much exposure. These ports are just too sensitive for any, I mean ANY outside plant exposure.

The hum you are picking up with your butt set and probe are just simple AC induction and difference in ground potential between the buildings. This is perfectly normal and is not the cause for the noise you are hearing on the phones themselves.

Sorry to bear such bad news, but I have spent years chasing these problems with multiple Panasonic customers and many thousands of feet of outside plant cable. It's always come back to hybrid ports being knocked out of balance.

Ed Vaughn, MBSWWYPBX