I have had the MFPs for years and just love them. I suspect that in order to get the fax machine functionality to work, you may have to connect to a traditional copper analog phone line. I had occasion to research this earlier this year, and it really depends on your specific MFP and the type of phone signal delivered by your broadband.
My DSL is through Verizon, and they came through and installed the FIOS fiber optic system last year. I was giving it a lot of thought, but discovered that the fiber optic phone signal does not work with most fax machines and answering machine systems. Verizon cannot tell you if their system will work with your devices. They tell you to buy the FIOS, plug everything in and see if they work. If the machines do not work, they suggest you buy new machines and see if they work. Since part of the FIOS installation involves removal of the copper line from the street to your house, you are hosed with no way to go back to a copper analog phone system. I was told that Verizon does the installation in such a way that you cannot put in a new copper line and revert back to analog phone service. I learned this by visiting some of the FIOS forums, where loss of fax capability is a common complaint. I had always thought that a phone signal was a phone signal was a phone signal, regardless of how it was delivered. I am apparently wrong on that.
My ex-wife got Comcast cable broadband/phone last year, and she can no longer use her MFP as a fax. Comcast has told her she is on her own with this problem.