you can use the voip that connect to your phone. to end user, it just like normal phone line. you can stretch it up to 5km if the VOIP convert the signal to standard 300-3000Hz.

if it comes with a phone line it should support fax

Yes it does, am using fax with unifi

phone and fax basically are the same thing. your phone convert electrical signal to voice, your fax convert it to bitmap.

your BTU by the way, convert light pulses to electrical signal.