I just switched from straight talk to Mint and I was setting up my iphone 6 s plus and it says " The sim card that you currently have installed in this iphone is from a carrier that is not supported under the activation policy that is currently assigned by the activation server. This is not a hardware issue with the iphone. Please insert another sim card from a supported carrier or request that this iphone be unlocked by you carrier.
Carl, thank you for your feedback. I did as you stated and called at&t and they had me get online and fill out a request for unlocking and a couple hours later my request was approved and I had my Iphone 6s Plus working with Mint Mobile Service.