Hershel,
Have you tried just putting a tone on the input to see if the other speakers get the signal? I don't see how having a stereo output would make a difference if the input is not so it most likely wouldn't make a difference; other than the volume being louder. It could be that they had a second page output, maybe through a trunk, and that was hooked up to the other speakers or possibly a zone controller if their page port allowed for DTMF.
Thanks,
TE