From US Cellular website (they are CDMA):
"What is the procedure for unlocking my device?
1X and 3G eligible devices may be unlocked at company owned or agent operated stores or by calling us at 888-944-9400. Because of the difficulty in unlocking some of our 4G devices you must bring these devices to a company owned store and we will either unlock the device for you or send it to a vendor who will perform the task and then have the device returned to the store. We hope to expand the capabilities of unlocking 4G devices to other distribution locations in the near future. You can call us at 888-944-9400 or your local U.S. Cellular store to determine the closest location to you that can handle your unlocking request. Your device may already be unlocked so be sure to check the unlocking status for your device listed above or by giving us a call. There is no charge for unlocking an individual device but U.S Cellular does reserve the right to charge non-customers a reasonable fee for any bulk requests for unlocking eligible devices.
How long will it take to have my eligible device unlocked?
Most devices can be unlocked on the spot but some 4G devices will need to be sent to an outside vendor to have the task performed. If the device needs to be sent to an outside vendor we will have the request processed as soon as is practical but you should plan on at least 3 or 4 days before the device can be unlocked and returned to the company owned store that processed your request."
This indicates you have to bring in a 4G device?
Honestly US cell customer service sucks. I went into the store and they said there was nothing they could do so I should go to apple. I went to apple and they told me to go to us cell. I called us cell tech support and they can supposedly send in a request, which is what I've done, twice now.
I know us cell is CDMA and normally their phones won't work with GSM networks, but the A1453 model 5s I have is the GSM model, it also has CDMA radios. It is internationally unlocked, I've used it on EE and 3, both GSM european carries without a problem. The device is locked domestically though.