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, Grandstream, 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.
An FXO paging device requires an FXO gateway (IP400 or Grandstream) to integrate to the PBX. You then need to create a new Outbound Route and add the trunk the paging device is connected to so the user can dial and access the overhead. Our experience has shown you want to be sure to strip digits in the trunk otherwise they may hear digits being dialed in the overhead.
Using an FXO paging device is the least advised method for a few reasons. The main reason is how you make the call. Instead of registering as an extension that can easily be dialed, you have to create an Outbound Route to access the gateway, meaning you cannot create a Ring Group with the paging device and some extensions included. Another reason has been our dealers experience with the FXO paging devices themselves. Sometimes special programming is needed that they are not aware of (eg. requiring an outside power source to give line voltage). It seems historically, the FXO overhead paging devices have been the most problematic so it is our advise to avoid them if possible.