My guess then is that the second pair is providing enough capacitance to 'absorb' any induced hum. This would often happen on paging adapters that were connected directly to CO lines. What we did in these instances was to connect a 'half ringer' module from a standard NID across the line to eliminate the hum. Keep in mind that inductive hum can come from the tiniest of stray voltages, especially when a line runs over long lengths of quad (non-twisted pair) wire. This drop wire is just that, shielded quad. With the sheath not grounded, it picks up just about anything out there like an antenna.
https://i.ebayimg.com/images/i/400465573437-0-1/s-l1000.jpg

Ed Vaughn, MBSWWYPBX