Traveling overseas this summer and would be nice to use this instead of my Xperia Z1s...but only if I can use a local sim card. Assume it should be no different for the code providers than the normal S6.
Search the forum, at least two people from memory; one using an online service, the other with an unlock code direct from AT&T (this option is available if you buy the phone for full price). AoN
I unlocked with AT&T after buying the phone outright - your only issue might be trying to update the phone, need an active AT&T sim card for that (until there are rooting/flashing options).
Bought the phone new from a reseller on Swappa. AT&T granted the unlock request in about 16 hours.
Using it on T-Mobile. Trying to confirm Band 12 LTE but haven't seen it pop up yet in LTE discovery. On band 4 I am getting 76.92mbps down, 26.64mbps up on speedtest...
I bought one on ebay and used cellfservices.com to unlock. I paid the extra $10 and got the code within 20-30 minutes. I had to change the APN settings from default to get LTE in socal, but works great now. Now if I could figure out how to get the July OTA update I'd be cooking with gas.