fax machine giving errors when trying to send

Recently I upgraded to the newest xfinity modem. Since then, I have had problems sending faxes. My machine is continually giving me a communication error. Again, this did not start happening until I installed the new modem. (note: I swapped out the modem for a second same new model and it didn't work either). I use a fax quite a bit for work and this has become a huge problem for me. I have gone through the xfinity troubleshooting and changed all of my fax setting accordingly and this does not make a difference. Other things I have tried: Swapped out fax with another machine, new modem, new phone cord, swapped power source to a different outlet. Still giving communication error with different machine. I have tried sending a fax to the same number from my actual office and it goes through no problem. Just not working at home using my Xfinity service. Please help!