I'm having trouble sending or receiving faxes over a VoIP (Voice over Internet Protocol) phone line. Why?
Because fax machines are designed to work over standard analog telephone
lines, Epson does not guarantee that fax transmission will work over VoIP phone
lines (cable systems such as Cox, Charter, Time Warner, Comcast, FiOS,
magicJack, and Vonage).
Internet traffic can interfere with faxing because VoIP communicates through the Internet and not through standard analog telephone line. The data sent over the Internet for a fax is digitized into packets and this process can cause data loss.
You can try slowing the transmission speed of the fax by setting ECM on the product to Off and selecting a slower Fax Speed. If this does not help, contact your VoIP service provider for help.