Problem: data doesn't work on 3G or 4G with T-Mobile on Nexus 5, and most calls drop immediately.
Data and calls work great if you change "Network Mode" from "LTE" to "2G". Oddly, SMS works in all modes and you can use MMI codes like #225#.
Solution: go to "Access Point Names" (APN) in Settings > More > Mobile Networks, and in whatever APN is currently selected (mine was "T-Mobile GPRS"), change the "APN Protocol" from "IPv6" to "IPv4". Make sure to save settings.
You can verify this by going to Settings > About phone > Status.
I'm not sure whom to blame (but I'm leaning towards T-Mobile since nothing changed on my end).
Yet another reason to stick to IPv4 :)
