Small Claims Court is your friend here. I am not an attorney, but a local attorney probably can advise you. Ask if you should pay the bill, follow the attorney's advise. In some jurisdictions paying the bill is admitting the debt.
Often calling and asking who is the proper contact for receipt of service tends to get their attention.
Send a certified demand letter that the charges be reversed, this shows that you have tried to resolve the problem, then go to the local court and file a small claims action. Claim all expenses, plus the bill and file against the local receiver of service.
The courts will take months, but Comcast will often try to placate you when they receive the notices from the court, be sure you get everything they offer in writing (it is useful in court.) Settle for no less than the full amount including court fees, they will try to avoid court, ask for a default judgment every time. Once you prevail, publish how to do this and your experience, others will thank you, but Comcast won't.
Oh, yes, if you inform them that their conversation will be recorded for "any and all legal purposes", they will be more likely to hang up, which looks bad in court.