PA-15 can connect to a TRUNK port if you set the switch to FXO

You can connect to an analogue station port set to IVR if you set the switch to FXS (the functionality is you dial the extension number of the analogue station, it rings one, and the PA-15 answers. Like calling an answering machine)

You can connect to an analogue station port set to paging port if you set the switch to FXS, AND add a Viking AIB-1 board (https://www.vikingelectronics.com/products/aib-1/)