I am in the VA/DC/MD area, and I live and work in NoVA.
My understanding, from Google searches and comments from friends, is that the 'Missing Calls While On WiFi and LTE' is a Sprint issue that has existed since the Nexus 6 debuted.
As for the lack of WiFi Calling, that is a completely separate issue, and is related to Sprint using some sort of technology, for their WiFi Calling, that was not the technology that T-Mo and other carriers adopted. So Google and Motorola decided to go with the most popular WiFi Calling technology.
The situation is somewhat similar to the situation when Sprint decided to go with WiMax instead of the more accepted LTE. My personal belief is that, just like the WiMax vs LTE, at some point Sprint will get on-board with the WiFi Calling technology that all of the other carriers are using, and then the Nexus line of phones will be able to use WiFi Calling on Sprint.