Don't know if you want to change out your page amp or maybe your present amp would accept a different source as well. I think Valcom and some other mfg's interface with a trunk port or maybe an analog port. If you have a spare trunk port, put that port in its own line pool....say dial 7.
Then you pick up the analog phone and dial 7 and your into the paging system. Or if it can use an analog station port, you pick up the analog phone and dial an extension-example x245- and you have access to your page unit. Just a thought. Check your present amp and see if it will take other sources other than Page output.
Of course if you end up replacing the amp make sure the new one works with the speakers or you will have to replace all the speakers, too.

If CON is the opposite of PRO
Then what is the opposite of Progress?