Verizon has a certification process, but they are effectively required by law to accept all compatible phones. Microsoft didn't even submit any of their recent phones for Verizon certification, even if they had chips which supported Verizon bands. Actually, HP is just now (like, this month) releasing a version of the Elite x3 that is Verizon-certified.

https://opendevelopment.verizonwireless.com/get-certified

https://opendevelopment.verizonwireless.com/device-showcase/...