Depending on your situation, you have the following options: * Request unlock from your wireless provider and follow their instructions. You might need to pay up the rest of your contract, etc. * If you do not know the locked carrier, find out by using one of the online IMEI checkers. * Once you know which carrier it is locked to, again follow Option 1 or use a remote unlocking service (with extreme caution). Notice, that these services may not be able to unlock your device. * If none of the above works, try using an unlock chip (Turbo SIM, etc). Do your homework first. * If this iPhone is iCloud locked (iCloud password protected) you are out of luck in either case and will need to contact the owner of this phone to help you remove it from their iCloud account. Useful Tip: Are you sure it is locked? Did you try using a different SIM card and received an error message saying "Unsupported SIM card"?
At&t's iPhone 5S will not work on Verizon or Sprint because it is not compatible with CDMA networks. The best way to check it is use Verizon"s BYOD page and type in the phone's IMEI to check if it can work on Verizon.
Actually, in most cases, it is impossible to tell which carrier the iPhone is or was used on by simply looking at the model number. Though in some cases it might work. There is actually an IMEI check which can show what is known as the "Initial Activation Policy" and this would be the original carrier of the device. Notice however that the same device can be used on another carrier later.
A Straight Talk sold iPhone 5S (and other ST iPhone models currently sold on their website iPhone SE, iPhone 7, etc) or any other prepaid iPhone for that matter, including Verizon's and T-Mobile prepaid smartphones will always come locked to that network. Here is a list of various prepaid networks operated by the four U.S. cellular providers. Unfortunately, you won't be able to use such a phone on another network until it is carrier unlocked. And depending on your prepaid provider, this may require from 3 to 6 or even to 12 months of active service on their network before your unlock request can be granted. The only exception as it seems is Tracfone. A locked Straight Talk iPhone will work with a Tracfone SIM card. Also, third party services are unable to unlock most prepaid iPhones. You can check the applicable policy on your carrier's website. Here you can find Straight Talk's unlocking policy. 'Source: https://attunlocker.us/faq.php