Does unopened fully paid iPhone need to be activated with AT&T sim card before using with another?

First of all, you need to ask where he bought the phone.  If he bought it from Apple, and bought the unlocked version without a SIM, then it should work.  If he bought an AT&T phone, it was sold locked to AT&T, and would have had to be unlocked first, and that seems unlikely if the phone was never opened or activated first.

I personally wouldn’t buy a new phone from some rando that I didn’t know, or without some method that I could be sure that I could get my money back if what they said wasn’t true.

The item was described as AT&T phone with both GSM and CDMA support,  so I think he bought it from AT&T. 

Apparently even if it was fully paid off, you need to use AT&T sim card to activate it and only after that you can use the phone with other sim card? Did I get it right? 

The phone is not locked yet. With Apple’s flex lock policy, it locks to the first carrier activated on. You should be able to activate the phone with any SIM card and the phone will then be locked to that carrier. If you activate it with an ATT SIM card, it will then be locked to ATT and you won’t be able to unlock it for at least 60 days and maybe as long as 6 months.

@sandblaster , thank you for the answer. 

What if I activate it outside of the USA? I don't live in the USA. Will it work? 

Also is there a way to unlock it if it is fully paid but I don't have access to AT&T in any way? 

The fact of getting locked to the first SIM card carrier sounds really bad, I would like to unlock it fully. 

Thanks

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@LibraM wrote:

@sandblaster , thank you for the answer. 

What if I activate it outside of the USA? I don't live in the USA. Will it work? I do not know.

Also is there a way to unlock it if it is fully paid but I don't have access to AT&T in any way? That depends on what carrier it is activated on and subsequently locked to. Only that carrier can unlock it. If possible, activate it with a Verizon SIM card. It will stay unlocked if activated on Verizon.

The fact of getting locked to the first SIM card carrier sounds really bad, I would like to unlock it fully. 

Thanks

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Since you don’t know exactly who the phone was originally bought from, there may be a different problem. If the phone was bought from ATT but does not get activated on ATT, it could get flagged as fraud and could get blacklisted, in which case it won’t work on any carrier.

Just a heads up, wanted to let everyone know, that we will never know, I cancelled the order.

Thanks

@sandblaster  Verizon locks phones for 60 days, under the US flex activation policy.