That's odd. Those comcast remotes are usually JP1 remotes with nearly infinite possibilities, much more functional than any sony. At least the comcast remote can usually do macros, volume punch through, key moves, key copies, advanced codes, favorites, mulit-macros, shifted functions, and even more if you buy or built an interface cable for it. The sony can do none of that.
HERE's a page detailing many of the things your comcast remote can do. And
HERE's another detailing what it can do with a JP1 cable.
If you still want to get the sony to work, just try all the cable codes (there are only 10 or so). If none work, you're out of luck.