Unlocking Samsung Galaxy s6 (from US Cellular -> AT&T)?

Prologue: Please forgive my noob ignorance on this topic.


So I picked up a Samsung Galaxy s6 (sm-g920r4) for a song locally, but it's US Cellular and my carrier is AT&T. I'd really appreciate some advice on the following...

1) Does it need to be unlocked?
2) If so, is this something AT&T can do?
3) Is there a tried and true method to unlock manually as to avoiding bricking the phone?

I've taken the US Cellular SIM out and set it up via WiFi as far as I can get.

Thanks in advance for any and all assistance.

It has to be unlocked from the carrior you bought it from. In this case us cellular would need to unlock it.

Unlocking the carrier on a smartphone is not something that would ever damage the phone - it has nothing to do with jailbreaking or rooting or unlocking the bootloader. Carrier unlocking is a one-time operation that "frees" a smartphone from the carrier aka network lock that typically is the one that sold it. Once a smartphone is carrier unlocked it can't really be carrier locked again in the future (it can by the phone manufacturer but they'd never do such a thing to be honest).

Contact the previous owner and ask them to contact US Cellular to have the device unlocked. If they can't do that (like the sale was done and they won't respond to you trying to reach them now) or they flat out refuse to do it then you'd end up having to use a third party unlocking service. The carrier unlock is typically done by a code you enter - if you insert a SIM card from another carrier into the phone it will usually trigger a notification asking you for the network unlock code aka the carrier unlock code. Some newer devices, especially those sold by T-Mobile and MetroPCS nowadays, can be unlocked with an app that comes preinstalled on the device.

You can purchase carrier unlock codes from many many places, but I'd do some checking around before committing to one - eBay has a lot of unlock code sellers, if you find one that specializes in the Galaxy S6 and they have a lot of great feedback (like 98% or higher) for their service they're usually worth the trouble but the actual pricing can vary quite a bit between the sellers so shop around if needed.