Originally posted by Curlycord:
When the alarm goes off they want to hear it over the speakers on the phones (via paging speakers on the sets).
My question back would be - why?
Phone systems and wiring aren't fail-safe and continuously monitored like fire alarm sounder circuits. They simply wouldn't count as part of the fire alarm system provision. If the existing sounders are inaudible then additional sounders are needed - not a linkup to a phone system which is quite likely to go out of use early on in a fire thanks to loss of power or damaged cabling.
Standards here (UK) require the same sound/tone to be used for all fire alarm systems on the same site, if your area is similar you may have to duplicate exactly - in synchronicity - the existing sounder tones.