How do I broadcast both sides of a phone call to overhead paging? Algo & CyberData say it cannot be done.

The application for a municipality is this: Outside caller dials a 10-digit phone number. The call rings multiple phones. Someone answers the call. And immediately both sides of the conversation is heard overhead through paging. So far Algo and CyberData say it cannot be done.

It would be great if I can use the existing overhead paging system that consists of amplifier and speakers/horns. But if I need to completely replace the overhead paging with IP units that is ok.