WiFi calling on a Nexus 5x using T-Mobile is not always available as a feature. That is, the feature when enables and set as a preference, does not always persist. T-Mobile WiFi calling when it is on works great. It's just not always staying on.
I have exactly the same scenario along with iPhones that are on the network in the house with no issues. Called Google Tech support who offered NO help whatsoever. T-Mobile is trying to lend some sort of assistance but remind me that this isn't a phone that they've provisioned. I then replied...