How do I delete a carrier on mini ipad?

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ATT says unlocked. Verizon says ATT has it locked. New Verizon SIM CARDS say SIM not valid. Carrier not supported. ATT SIM cards say device unlocked in the device unlocked website for ATT. I have been trying since October to get 2 mini ipads away from ATT and to work with Verizon. ATT told me yesterday I needed to get Apple to unlock the pads. I called Apple only to be hung up on and the guy never called me back. I hate ATT, frustrated with crappy customer service. Anybody know how to make the Carrier in Settings change?