back in college i had a first generation Tivo that would only get updates via dial-up. I didn't have phone service so i used vonage through comcast cable internet service and it worked. Kinda the same principal i guess.
I still fax occasionally with my HP all-in-one printer and we have Comcast. You may have problems if you have too many splitters or a weak signal at the jack you use for your cable modem. As long as you have a good signal there, you won't have problems. If it is weak, Comcast will run a line straight to that jack for you (at least they did for us).