Do you have Comcast Digital Voice? If so, and the cable modem is properly hooked into the house's phione system, you simply plug a regular phone cord from any house phone jack and connect it to your phone and/or fax, just like you would normally. Othwise, simply use a phone line from the phone jacks on the cable modem to your phone or fax.
I had a DSL cable filter/splitter that I thought I no longer needed. Turns out I do need to make my phones work. I plugged the phone line into the back of the modem. I plugged the DSL phone line into the PC. I plugged the phone itself into the PC. All works. Except the FAX. I will take someone's advise and program the PC to send the FAX.