@lutorm 's linked read has everything you need to resolve this.
Just be professional, write those letters, and keep and organized paper trail and notes with the names and dates of anyone you happened to talk with on the phone. The phone is really not your friend, though . . . letters are. Email can work sometimes and does create some sort of paper trail, but only if it actually goes to someone with an obligation or inclination to resolve you issue. Letters through the postal service are much, much better. Send bland letters first that include the relevant facts only (dates, amounts, account numbers, etc), the then kick those up the food chain in a month or two. Unfortunately it will likely take a couple of months, so be patient. Trying to go up the food chain too fast might not work, but if two months has gone by since your first letter, then persistent additional letters to higher ups will probably do the trick. You probably should contact US Cellular along the way as an initial written documentation to make sure they are informed that an account was fraudulently opened.
I've had the letter and then escalation strategy work multiple times with multiple banks who weren't doing what they should be doing. In the most recent one, I followed up 60 days into the process by attaching pdf copies of my letters to a brief and friendly email to a CEO of a gigantic bank. I was able to correctly guess his email address, which I think is funny, but corporate email addresses often aren't difficult to guess if you have email addresses of other specific people at the same company.
It was a short email, and opened by thanking him for a recent large donation to a nonprofit I work for (news I found when searching for his address online). During my googling, I also found out that he hadn't had his new job for very long, so here is the opening of the short but key middle paragraph of my email:
"Trust is the very foundation of the banking system. Customers trust the company you lead to handle their money and transaction properly and promptly. I am cc’ing you on a letter outlining a case in which that trust has been violated because I think it is particularly important for leaders in new positions to understand . . . yadda yadda yadda" I then ended it with "Thank you for your consideration, and please let me know if you have any questions or need more information.
With kind regards, Zamboni"
This was for a mistake a bank made to the tune of $3800. They had been "investigating" for the past two months. I sent a couple of letters and tried the phone route several times, and each time just sent me an infuriating form letter concluding that they had acted properly and didn't owe me $3800. So, I sent that email with the attached letters to the CEO in the morning and was contacted by someone that very afternoon who was suddenly urgent and efficient and extremely polite and she had the matter resolved entirely by FedExing me a check that I received two days later. It's likely that the CEO never even saw my email. But someone who wants to keep his hot breath off their neck screens his email, and they made sure it got resolved so fast that I wouldn't follow up with him again.
Good luck with it all. What a pain! But you can do it!