AT&T Next Plan and Phone Resale

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@tcgibbs wrote:

I currently have an iphone 6s 16 gb under the Next 24 plan, but a friend of mine was selling her iphone 6s 64 gb and I bought it. Am I allowed to transfer my sim card over to the new phone and start using that and sell my previous iphone even though I am still making payments?

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Yes and yes.  

You can sell it (it's your phone), but be sure to tell them it's locked to AT&T (or GoPhone).  

Once you pay it off you can unlock it.

Yes, you can do that. You are under no obligation to use the phone you are paying for. It's your phone, you can do whatever you want with it, including selling it. All ATT cares is that you continue  paying for it.