Replace the phone cable and or disable ECM mode.
Do you mention the phone company /router because it started happening after you switched to those things?
ECM mode is disabled via System > Admin Menu > Fax Settings > ECM
In the same menu set replace ECM with Modem Speed and attempt it at a lower setting than whatever is current as well.
Persistent Error 035-705. Docs scanned fax on machine. It dials number, attempts handshake, sounds like it connects, but then fails.
Fax confirmation page shows Mode:ECM and Result: 035-702 and documents not sent.
These numbers are current and working per contact with them,
Have attempted power off/power on reboot. Checked phone connection.
We use Xfinity for phone, and newest edge router -
Everything working just fine for the past year.