Thanks for the suggestions. I have a bench supply with which I can set voltage and limit current with a pot, so I may take that approach. Rig up a safe breakout box so I can access the pairs reliably. Then hope that PEI was either careful about being consistent with polarities, or careful about protection against reverse polarity. Continuity testing should tell me about diodes.
I suspect there are connections, at a minimum, for common audible (Sonalert), constant power for dialer memory, and constant power for button release solenoids. Now to make space and make time to experiment...