There are three possible methods to integrate overhead paging to the PBX; SIP, FXS, or FXO.
Using a SIP based overhead paging device is the best option as the device registers directly to the PBX as a SIP extension. This allows for the easiest integration as we do not need a gateway between the PBX and the paging device. In order to page, you will dial the SIP extension, and since it is registered as a SIP extension, you are also able to add the overhead to a paging ring group, along with the other extensions on the system.
An FXS paging device requires an FXS gateway (IPr20, PAP2, Granstream, or SPA112) to integrate as a SIP extension and is the second best option. Again, you would dial the extension to page, or you can add the extension to a ring group and page overhead along with a group of phones.