Can't VOIP outbound fax to work since switching to Xfinity

I previously used an OBI202 VOIP adapter connected to a DSL modem and my Google voice number to send many faxes some of which where many pages.  Never had a problem.  When Xfinity was installed on the street I switched from DSL to utilizing a customer owned Arris SBG6950AC2 with 25 Mbps service.  Everything works fine except for faxing.  Using the same network configuration (a 10 dot) just can't get a fax to go through from either my new HP7740 or my Epson ET-4750.  They both dial and there is an answer but then get "Communication Error).  I also hooked up my antique fax and it won't work either.   Note that voice calls work fine and are clear.

Since it's a customer modem I have some access to change things other than firmware.  Can I ever get this to work or am I "beating" a dead horse?  Really want to continue to help my neighbors and myself send faxes.  Suggestions?