I had this problem in an older house I owned once. I put the wires
from each circuit to spade lugs. Then, I could easily slip one off of
the binding post screws for testing. I also had a spare jack screwed
on to the mounting board that was connected to the outgoing phone
line. Easy to break out a circuit and test things. The newer service
box allows you to disconnect each circuit, more or less, which
addresses this problem nicely. A trip to Rat Shack??