If it's coming from inside your computer, it sounds as if you have a modem device set to "auto-answer". This doesn't have to mean it's a fax. The sound might be that of rising/falling tones and white noise that occurs during the connection negotiating process. Look for modem in device manager. There would be several ways to deal with this. If you don't have a telephone device connected to the PC, you can disable the modem. If you do, you should be able to disable the auto-answer function.
I have my FIOS modem hooked up to the desktop. When the phone rings on the 4th ring it sounds like its going to a fax. I don't have a fax machine. I unplugged the answering machine but it does the same thing. The only time it rings more than 4x is when the FIOS is not hooked up to the computer.
I'm not sure which forum I should be in. I called FIOS, Dell, V-Tech (answering machine) and none can help. They insist I have some type of fax software installed - I do not.