They can't stop you from using a device that technically works on their network. Say for instance you bring an unlocked AT&T iPhone 6 to Verizon. If you have a

They can't stop you from using a device that technically works on their network. Say for instance you bring an unlocked AT&T iPhone 6 to Verizon. If you have an existing sim card (all LTE devices have them even on Verizon), you can simply swap your sim card into the device and it will activate automatically and work just fine. However, if you take that device into the store and need to get a sim card, they will refuse to give you one as they only activate devices that are on their whitelist. A way around it would be to just borrow someone else's Verizon LTE capable smartphone to activate temporarily to get a sim card and then just swap it into your device you want to use.