T-Mobile said as of late last night Pacific time, the service was fully restored and customers were able to start calling and texting again. But even though it was able to fix the problem, it continues to investigate the root cause. In a statement, the company said: “About five percent of our customers across various geographies were affected for much of Tuesday evening, and by late Tuesday PST their service was restored. Our sole focus has been restoring full services for all customers; we are now investigating the root cause of the incident. We sincerely apologize for the inconvenience that this has caused our customers.”

UPDATE: In a statement, T-Mobile said Wednesday evening: “After investigating the cause, we have determined that a backend system software error had generated abnormal congestion on the network. T-Mobile has since implemented additional measures to help prevent this from happening in the future.  We again apologize to those customers who were affected and may have been inconvenienced.”