Account cancellation and refund

I moved last April and had my daughter cancel our cable service. It took a while but it finally worked. The only problem is that Xfinity has continued to bill for a smartphone that was NEVER activated. Again, my daughter called and walked into two different Xfinity stores to resolve this issue. At one point we were given a $50 Refund card, but the account continued to bill for the inactive phone. That phone was my husbands and he's been dead for 10 years. I asked for a refund and one store manager said okay, we've disconnected your credit card from the account. STILL, XFINITY continued to bill rather than give me the refund. NOW, today I received a threat that my unpaid bill will go to collections if I don't pay it. WHO can be held accountable for this horrible service, and more importantly, HOW will you resolve it?