One thing you can try is, take a regular old analog phone (remember those?) and plug it into the fax port and call it.
Does it ring the phone? - If yes, check your fax machine settings.
If not, can you answer it anyhow? - if yes, a reboot may help but it may also be a problem with the ring voltage (hardware).
If you can't answer it, then it's probably not ringing where you think it is. Using the phone, call an internal number and verify the extension. If the extension is good, check the port for any call forwarding/rerouting or to see if it's actually ringing into your system at all.
Ralph