It sounds like you believe that this negative record is wholly in error -- that you have never had an account with Comcast before. Is that right?
It sounds like you are certain that the original creditor was Comcast, though it is unclear to me how you know that. Is it because you have spoken with the collection agency and they told you that? Or have they mailed you something that states that the original creditor was Comcast?
Usually the best thing to do in these situations is to gather as much information as possible on your own, before disputing it. (Assuming, that is, that you are not close to the Statute of Limitations, in which case it may make sense to lie low for a while until you are certain you are well past it.) You can often find out how someone else's account got mistakenly connected with you by doing a little legwork. You find out the account number for the Comcast account, the address on the account that Comcast was servicing, the dates of service, the exact name on the account, any phone numbers attached to the account, etc. Comcast may be able to give you a lot of this, once you get as much as you can from the CA. You may be able to show quite easily that you were living in a different state at the time -- or perhaps that the account belongs to a person who moved into the same apartment as you a month after you moved out.
But just issuing a blanket "dispute" will result in the CRA contacting the CA who will say that the debt is valid and then the CRA will tell you that the account has been verified. Alll you have achieved is used up one of your disputes, and you can't keep disputing an account without the CRA dismissing future disputes as frivolous.
You may want to try doing the detective work I suggest above to try to figure out how it is that an account you are certain belongs to someone else was mistakenly attributed to you. You may also consider pulling your report at CRAs that specialize in telecom accounts, e.g. the National Consumer Telecom & Utilities Exchange (NCTUE). That may give you some insight into what happened, When I pulled my NCTUE report I found a telecom account on my report that belonged to someone else. (The guy had moved into my old apartment so we shared a common address.)
