Even with Verizon if you have a phone which supports their LTE bands, you can use them on Verizon. I used my iPhone 6 and iPhone SE from AT&T on Verizon.
Both SE models support CDMA and GSM but different bands. But only some iPhone 6 models support CDMA.
http://www.everymac.com/systems/apple/iphone/iphone-faq/diff...
And then there is another scenario like I believe some of the Google Nexus tablets only support LTE on Verizon and won't work at all if there is no 4G on a CDMA network. It will work on 3G/3.5G GSM networks.