Disputing a cell phone bill - Collection agency involved

[KostnersindaHole]

So I have been dealing with this for the past month now and it is getting super frustrating. Here is the problem:

I used to have a Sprint account and closed it a few months ago as my company is now paying for my phone and plan. I had a final balance, so I called up at the end of October and paid over the phone. I saw the charge on my credit card statement and thought everything was fine.

About 2 weeks ago, I started getting phone calls from a collection agency saying I haven't paid the balance. So I call Sprint and they are still telling me I owe them the money. I explain to them that I have the credit card statement showing the charge and a confirmation email from them showing that they received my payment, yet they are telling me that I didn't pay. Every time I ask them if I can fax over my proof, they tell me that they will open a case about it and someone will contact me regarding the issue which hasn't happened yet.

In the meantime I have been getting a call from the collection agency every day about this balance so I have to explain to them that I have already paid the bill, which is incredibly frustrating, plus it takes about 15-20 minutes out of my day at work.

Is there anything I can do to solve this problem? Every single person I call and talk to is completely incompetent. They won't give me a fax number or address to send the confirmation email/credit card statement to. They just keep telling me they'll open up a case, which seems to go nowhere. What else can I do?