I totally agree with Dave's assessment. You really have no way of isolating your phones. Assume that the phones have a single demark point home-run point in your apartment, maybe you do.

Look at all of the jacks and clean them up. Look for signs of moisture.

if it happens again, check the laundry room. See how bad the "dew point" is.

Here http://protective.sherwin-williams.c...on-chart_f.pdf is a Dew point chart. You have temperature, relative humidity and surface temperature.

Once you calculate the dewpoint, the surface temperature should be higher than this. The surface being the punch down block surface.

While your in that box, arrange your wires so they have "drip loops" on them. You don't want water(moisture) traveling along wires and getting the terminal area wet.
Cleaning the area between your terminals should be sufficient for a while anyway if that's the problem.