I haven't seen a product that does this. Best solution I have seen is XMedius Fax with seperate number, does T38 with dial peers on gateway. All software based solution. Whatever fax solution you go with I'd make sure it does T38 software only, no hardware.
I sometimes forward my desk phone to my cell phone, I'd go nuts if my cell phone rang and it was a fax. It would retry 3 or more times, etc.