I've heard of dual SIMs but not SIM Free. I was on the Microsoft Store site in the MS Offer's tab showing AT&T's black and white selections when I noticed it. I probably have missed something here but figured I'd ask...
https://www.microsoft.com/en-us/mobile/phone/lumia950/offers/
MG
https://www.microsoft.com/en-us/mobile/phone/lumia950/offers/
MG