Mostly a knowledge dump for those searching for it, including possibly my future self.
Default config doesn't work right. VoWiFi is greyed out, and Advanced LTE voice is grayed out. How to fix?
Choose "Phone Information" and under "Set Preferred Network Type" choose "LTE/UMTS auto (PRL)". LTE should provision and in the corner triple dot menu, choose "IMS Service Status" and if it now says "IMS Registration: Registered" you're good to go. If not, contact T-Mobile and tell them you have a problem with your Nexus 6p and IMS provisioning and ask them to check the knowledge base, as they need to open a ticket with their NOC to fix it. There's a known issue with the IMS hostname or something like that. It'll take a few days to fix. Try this again if it doesn't resolve itself in a few days.