By Colette Bennett The war between mobile carriers is heating up. While there’s nothing new about one public-facing company issuing a challenge to another, being called out in the era of social media is an all-new kind of beast. In fact, many brands have made a game of it on Twitter, often to hilarious results. But those are often of the ribbing variety, more in good fun than any kind of actual challenge. And then there’s what happened with T-Mobile (TMUS) – Get T-Mobile US, Inc. Report this week. On May 31, the mobile carrier–or, as it calls itself, the “Un-Carrier”– sent out a release that…