Do you mean you have comcast digital voice? If so you should be able to just plug right into the modem's phone jack (or any other phone jack that connects to it) without any filters. DSL filters are only for DSL service.
Though I'm not positive if there are any issues with running fax via comcast's VOIP service. I think it works fine.
after searching for hours on the internet today I discovered a simple trick. Faxes only use the middle two pins(red and green) of the newer 4 pins connectors. The outer two pins (yellow and black) create a host of problems including dropped faxes, communications errors and "phone off hook" messages. Disable the outer two and your fax will work like a charm plugged into the cable modem or using a splitter for the phone!
