David Lesher wrote:
Agree corroded wiring connection as likely cause.

Go to each jack and put a *milliammeter* across Tip-Ring.
Start at the demark.

The milliampmeter should be in series with a 400 Ohm resister. The
resister represents the resistance
in the phone.

Bill K7NOM
You should see 20+ mA. If you don't, it's the wiring or jack.

As a next step, put the meter in series with a phone, and see if the off-hook
current is still 20mA.

The first indicts the upstream wiring; the 2nd is likely the phone.