Ask for details on their phone system.
While we have TAPI plugin functions for Windows, that's quite old.
Newer phone systems may have a web service and are accessible via local network.
So you can use our MBS Plugin for the HTTP request (CURL functions) or for direct sockets (TCP Socket functions).
I just did the Filemaker end of an integration project with Mitel software and IP phones. We trigger the software with Base Elements and the command line.. simple and free.
The clever part is that they work on Amazon workspaces, so the software is remote, but the phone hardware is local and attached to a zero client.
We do phones and FileMaker and the limitations are more often on the phone server side. TAPI is one answer but most IP phone system companies are now using the integration an APIs to squeeze a few more $ out of their clients.
A lot depends on the phone system some are easier than others.