Nighthawk X4S AC2600 - Can’t connect landline phone

> [...] I have just connected a Nighthawk X4S AC2600 WiFi VDSL/ADSL
> Modem Router to my Australian NBN System (fibre to the home). [...]

   "connected" it to what?  How, exactly?  DSL?  Ethernet?  To which
hole?

> [...] I have tried different combinations and directions with the
included splitter/filter but no success. [...]

   None of that is a useful problem description.  It does not say what
you did.  It does not say what happened when you did it.  As usual,
showing actual actions (commands) with their actual results (error
messages, LED indicators, ...) can be more helpful than vague
descriptions or interpretations.  "directions"?  "included
splitter/filter"?

> [...] I know it should work because with the previous modem (Telstra
> Smart Modem) it worked perfectly. [...]

   Not a very detailed description of that device, or how you used it.
And that was with the same service from your ISP?  And any two
router-like gizmos are always functionally equivalent?  You "know" more
than I do, based on no useful information.

> [...] I was wondering if I might have missed something in the setup
> online. [...]

   And you came here because you expected the psychics to know what you
did with your mystery devices, following your invisible "directions"?