It's been a rough morning for T-Mobile. Perhaps you've noticed your phone has been unusually silent, and people don't seem to be responding to your text messages. Well, the reason is probably the extensive outage Tmo has been dealing with for the few hours. That, or no one likes you. Either way.
— DownDetector (@downdetector) June 6, 2014
The problems are widespread, but not universal. It seems to be affecting data connectivity in some cases, but mostly voice calls. Affected areas are failing to connect any call, instead playing a recorded message saying that the number is unavailable. Judging from Tweets and other sources of complaining, the issues seem most prevalent on the east coast.
Nothing to do now but wait it out.
Update: T-Mobile has responded to Tmo News about the outage:
The T-Mobile network team was able to quickly resolve call processing issues affecting some customers briefly this morning. We apologize to our customers for any inconvenience.