NEW YORK (CNNMoney) -- T-Mobile USA's CEO, Philipp Humm, resigned his post Wednesday after two years on the job.
T-Mobile, the fourth-largest wireless carrier in the U.S., said Humm "is going to pursue a career outside of Deutsche Telekom" and reunite with his family, who lives in Europe. Deutsche Telekom is T-Mobile's parent company.
Jim Alling, the COO of T-Mobile USA, will serve as interim CEO.
A report in the Wall Street Journal said an internal company memo told staff that Humm had been planning to leave in September to rejoin his family, but that the timetable was sped up after he informed the company that he is joining a Telekom competitor.
A T-Mobile USA spokesman directed a request for comment to Telekom in Germany, whose representatives declined to comment on whether Humm will be working for a competitor.
"Philipp Humm has given the company some important initiatives over the past years," Deutsche Telekom CEO René Obermann said in a written statement. "Now we need somebody who can convert initiatives into market-successes."
Humm's resignation is unexpected, but the two years he spent at the helm were a rough stretch for T-Mobile. A proposed merger between AT&T and T-Mobile fell through last year.
AT&T offered $39 billion for T-Mobile, a deal that would have made AT&T by far the largest U.S. wireless carrier. AT&T said the buyout was necessary in part because of a nationwide spectrum crunch.
But U.S. officials weren't buying it. Both the Federal Communications Commission and the Department of Justice moved to block the deal, citing a laundry list of concerns. AT&T withdrew its bid in December.
T-Mobile scored $3 billion as a breakup fee from AT&T, plus another $1 billion worth of spectrum.
That gave T-Mobile a boost, but the company doesn't have enough spectrum to deploy a competitive 4G network. The carrier is expected to merge with someone else -- if it can find a buyer able to pass regulatory muster.