Service provider
I have an iPhone X and I just upgraded to a new phone. It was previously set up with Verizon. Can someone else use the iPhone X now but with a different provider-AT&T?
iPhone X
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If the iPhone X is unlocked. Only you could get it unlocked if it isn't unlocked. If the iPhone X is still in your possession, turn it on, go to Setting>General>About>Carrier Lock. If it says No SIM Restrictions, then it is already unlocked. If it does not read that, then you will have to ask Verizon about unlocking it. See this, How to unlock your iPhone for use with a different carrier - Apple Support
Once you get it unlocked, follow these steps, What to do before you sell, give away, or trade in your iPhone, iPad, or iPod touch - Apple Support, then someone else can use it.