Over the last week or two a fairly serious Android security flaw came to light called StageFright which makes it possible for hackers to attack Android users through text messages. That's a fairly short and straight to the point description of the vulnerability, but there's quite a bit more to it than that. Because of the potential severity of the problem it had users wondering when and if the manufacturers of their devices would be sending out updates to patch the problem. In Google's case, they began rolling out the fix to their lineup of Nexus devices just yesterday. Not quite directly though as the new firmware build labeled LMY481 was pushed to AOSP alongside the factory images which Google posted up to their servers, but no OTA's.
Now, for the most part, factory images can be just as good as an OTA update if you're an individual who has the know-how to flash the software to your device. In many cases yesterday this is what people have been doing, but it seems that the factory image build has been messing things up for some Nexus 6 owners, specifically those using the device with a T-Mobile USA SIM card inside. Multiple T-Mobile Nexus 6 users report having their device being cutoff from the cell network after flashing the factory image to their device. This of course presents a fairly big issue as they can now no longer utilize anything that requires a connection to the cellular network, which includes voice calls as well as messages.
Since this seems to only be affecting those who moved from firmware build LYZ28E which was T-Mobile's specific build with the WiFi calling radios, other Nexus users who weren't on this specific build are probably OK. T-Mobile also states that they'll be sending out there specific StageFright firmware update for the Nexus 6 which will end up being firmware build LYZ28J which has the WiFi calling specific radios included. The software should end up hitting users' devices soon, so for now those who haven't flashed any new firmware will want to wait it out for T-Mobile to shove things through. Those who flashed the factory images from yesterday and are experiencing connection issues would benefit from flashing back to the firmware build they were on prior to flashing to yesterday's new build.