I had a friend that had this exact same issue. He canceled his service and received a bill for the total that was left owed on all his devices. He kept making payments in the amount that the device payments were each month. That was 6 months ago and his account was never sent to collections. The phones will not be blacklisted but only the Pixel and Iphone's are fully compatible with other carriers. Other phones will work on other carriers but may not be able to get all there LTE bands or only be able to get there LTE service since Verizon uses CDMA for voice.