Are you calling the Comcast Cable Card line? 877-405-2298
TiVo has a Cable Card Hotline for setup issues:
I have read that if you are unable to install a cable card or Comcast is having issues activating it, exchanging the cable card for a fresh one can help in some cases.
TiVo Roamio Pro Lifetime ∙ TiVo Mini Lifetime
XBOX LIVE: fiftyfootfang