Collections notice for fraudulent charge

Back in June, someone used my Xfinity account to set up Xfinity mobile service. I was never at risk for the scammers stealing my money, because they used a card that is in no way connected to me. I called customer support, they disconnected the lines, and then received a voicemail saying the ticket was closed and there was nothing else for me to do. Recently, I received a collections notice for the outstanding balance. I suppose Comcast charged the card, and of course, it declined. I contacted the collections agency and their solution was to pay the balance to “get it over with”. I then contacted Xfinity Mobile support and their solution was for me to pay and then contact them again for a refund (maybe). Why would I be expected to pay for a service someone else used my information to sign up for? Secondly, why would I be expected to pay the balance because a stranger’s card keeps declining?