Fios DV works fine for me for voice & fax out of the home office, with a single paired wiring (check it for reveresed polarity ??) - old DSL filter removed (unlikely to be the source of your problem) Tried a different RJ-11 cord with 2 wired pairs ?? Temporary disconnect other devices on other outlets while testing.
Assumed that this fax modem was working fine before on analog signal, does it work on incoming fax calling - try to have someone dial your # with it enabled for auto-answering mode.
Reference the good old Hayes "AT" command set - »en.wikipedia.org/wiki/Hayes_comm ··· mand_set
- you can take a look at its status after running "ATZ" with "ATV"
If this is on a desktop PC, try swapping out to a different fax modem from an older PC since newer builds no longer bothered with it, plenty of cheap & good ones on eBay.