AT&T has unlocked my old iphone 3gs IOS 6.1.6 ... BUT I am unable to use it on your system because they have also blocked access to Cellular Data Network. I cannot modify the APN, etc. to make US Mobile's SIM card work even though the device is showing that SIM number under ICCID. US Mobile was unable to send the codes needed. I am now still trying to figure out how to get access to the APN. Supposedly carriers can access and send codes over the air. I will try Apple support and AT&T and hope one of them can help. But, for the low cost of US Mobile, I am appreciative of your desire to help. Luckily, I have another, older IOS 4.0, iPhone that is working with US Mobile but it won't connect to iCloud for me to upload over 400 contacts. I don't want to have to do that manually. Thank you for your services. Best Wishes for future our successes. ^_^