Originally Posted by AnalogMan

The device stays connected to LTE, but gets zero data. I didn't check closely, but my tablet knew the data connection was no longer working - either whatever sever it's pinging implicitly to confirm it's connected to the internet is no longer reachable, or the device is losing it's IP address. The former is more likely as I believe the t-mobile web site is still accessible (presumably so people can pay for more data, unlike AT&T, which didn't even allow access to the web site to buy more data).

T-Mobile also sends an email that's fairly unambigous: "You've used all 100% of 4G LTE data in your Mobile Internet plan and no longer have data on your <my device type information removed> until your plan renews."