You should be able to ditch the PagePal entirely, as the ZPM-3 has an input for a Tip/Ring "Night Line" for the ringing over paging.

Interface from the Magix paging trunk to the ZPM and configure as a loop start trunk. You may need a PRS-AS-A power supply to provide talk battery.

Configure as if you were using all self amplified speakers, except that for building 1, the output of the ZPA goes to the input of the amplicenter instead of the input of a self-amplified speaker.

You may need to jumper the contact-closure input on the amplicenter to keep it in the page-enable mode permanantly.