There's a confirmed T-Mobile service outage affecting users nationwide. People are reporting that all calls are failing with busy tones or getting dropped instantly, despite full signal bars. Let us know what you're experiencing and where you're based. Updated.
As Twitter and forums are filled with annoyance over the issue, T-Mobile is assuring us that they are aware of the problem and working on it:
T-Mobile customers may be experiencing service disruptions impacting voice and data. Our rapid response teams have been mobilized to restore service as quickly as possible. We will provide updates as more information is available.
We're waiting on further information from T-Mobile and will update as we learn more.
Update 1: Several commenters are reporting that both incoming and outgoing calls are working if routed through Google Voice.
Update 2: Many reports that T-Mobile @Home is out as well.