I hated dealing with Comcast at a job where one of the things I had to do was to get off spam lists.

Getting on the list can be as easy as some set number of comcast people clicking on something that came from you as spam, and poof, you are on the list. Even worse, if you are using a shared host for mass mailing (like constant contact or something) or a domain host (like namecheap) they will block everything from that host even if it's not anything to do with you.

It's not going to be easy, and it's probably not going to get you to a human any time soon, but don't give up. It's been a few years since I've had to deal with them, but just keep trying.