 T-Mobile US had its first big network meltdown since leapfrogging ATT to become the No. 2 wireless carrier in the U.S., and the companys response has been uncharacteristically quiet. As many as 68 million of T-Mobiles wireless customers had their service disrupted for several hours Monday as the company tried to identify the problem and restore service. The company said the outage was caused by router issues. T-Mobiles parent company, Deutsche Telekom AG, said Tuesday that it was related to 5G network upgrades. But the carrier is still sorting through the chain of events that caused the breakdown and has yet to provide an update. In the meantime, the Federal Communications Commission has vowed to investigate the matter. Its the first big test for the new company, which absorbed Sprint Corp. in April after a two-year battle for regulatory clearance. And it puts the companys new chief executive officer, Mike Sievert, in the spotlight -- though, for now, he has yet to comment on the causes of the problem. He has retweeted remarks from his technology chief and told customers on Monday that he appreciated their patience while the company worked on the issue. The T-Mobile network outage is unacceptable, FCC Chairman Ajit Pai tweeted Monday evening when he announced the agency would launch an investigation. Were demanding answers -- and so are American consumers. At 1:03 a.m. EDT, T-Mobiles technology head Neville Ray tweeted to say service was fixed. The timing is especially awkward for T-Mobile, since it just spent two years assuring regulators that joining forces would Sprint would bring a robust network. But its not uncommon for the FCC to probe outages. Regulators look for ways to prevent future problems and hand out fines for some mishaps. After a... 