iPhone 6 won't activate

[Nana08]

Hi! I'm not a techie at all and I really need some help. I have an iPhone 6 that gives this message:
SIM Not Valid. 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 your carrier. Please contact Apple for more information."

I have spent HOURS on the phone with both Apple and AT&T. I have an AT&T Go Phone nano sim in the phone. Is there anyone out there who knows what is going on or what to do? I can't get past this screen. Thank you all.

Nana08

 

[sirghost]

That message is coming up because the phone is locked to a different carrier than the one your sim is assigned to. You need the imei number which you can get form the box and look up who the carrier is so you can get it unlocked, or contact the original owner and find out what network he had it on and have him unlock it with the carrier.

http://www.imei.info

 

[Applejuiced]

Phone is locked and won't work with the carrier of that sim.
Where did you get it? You know what carrier it's locked to?
Is it supposed to be unlocked?