Update: Google has officially acknowledged the issue, stating:
Thanks for the reports everyone. We are investigating this issue. I’ll post back here if we’d like any additional data from the community.
As others have mentioned in this conversation, you can work around this by turning your phone off and then back on again. I know that’s not ideal, but I want to be sure and mention it in case you find yourself in this state.
(I’m going to mark this post as a best answer to this question. Please don’t take that as a sign that Google considers this issue fully resolved. Rather, it’s the best way for this forum system to display an important update at the beginning of a thread, and I want to help ensure that any newcomers to this discussion see the latest update from me.)
The Nexus 6 from Motorola is meant to be Google’s flagship phone in the Nexus line at the moment, and as such, it gets a fair amount of coverage when Nexus devices are discussed. This is usually a good thing, except when there’s an issue, and there appears to be one. At present, a number of users (seemingly US based, but it doesn’t appear to be limited) are reporting mobile connectivity failures, on a number of carriers, software versions and across a number of locations.
The issue can be observed almost at random when the 3G or LTE connection fails, and the signal bars display with a ! logo imposed over the top. This seemingly occurs whether there’s Wi-Fi active or not, and whether there’s a plausible connection to the local 3G or LTE tower. Rebooting the Nexus 6 seems to fix the issue for a short while, before the issue simply comes right back. There’s no permanent fix in sight.
The problem seems to be affecting T-Mobile and Sprint network customers for the most part, though at least one XDA poster on Verizon and another in the United Kingdom say they’ve experienced the same issue.
If you’re a Nexus 6 user in Australia, please let us know if you’ve seen this issue occurring, and the circumstances in which you’ve seen it, and we’ll try and push Google for any word on a fix.