To make a fax machine possibly work with VoIP you need to slow down the send rate quite a bit (found in settings on any decent machine). This will in effect try to slow down the sending process to that of a standard analog line. But you may still incur errors and issues. Easiest fix is E-Fax, its available in your control panel using your existing number, or you could use a cloud number to send and receive faxes, my business operates with E-Fax alone. Won't ever go back.