Failing Profile and PRL update

[Robin Shope]

As of yesterday, my husband's Nexus 6P would not make a call. It would get a message saying "your device is not yet activated". Spent over an hour on the phone with Sprint. They kept insisting that there should be a "more" selection in the settings. There is not. They tried to remotely reset the profile as well as had me dial a series of numbers. Nothing worked. Now what? Where it's this mystical," more" in settings? Why isn't there an option to do this task?
This phone was bought at Best Buy, over 2 years ago and had worked fine until yesterday. It is not a Sprint phone but used on Sprint network. I believed that means it is unlocked.