If you want to use an analog line and internal extension number for access to an external paging system, you need a "station level page adapter" like a Valcom 9940 (or whatever the current equivalent is).
The paging adapter detects ring voltage, answers the line and cuts the voice thru to a 600 or 8 ohm output which you then connect to the input of a PA amplifier.
Check with the manufacturer of the PA system.....they may have what you need in their product line.
Good Luck,
Dave Strang