T-Mobile US Inc. CEO Philipp Humm unexpectedly resigned from the fourth-largest U.S. operator Wednesday.

Chief operating officer Jim Alling will take the helm at the service provider for the time being. T-Mobile's parent company Deutsche Telekom AG (NYSE: DT) says that the search is underway for a successor to the exiting CEO.

Humm has been with T-Mobile since 2005. In November 2010 he took over for Robert Dotson to become CEO of the company's U.S. operation. Humm now plans to seek an outside career so can move back to be with his family in Europe, according to T-Mobile.

"Philipp Humm has given the company some important initiatives over the past years: Under his leadership the cost situation at T-Mobile USA has vastly improved, and he led the company during a difficult phase regarding the planned merger with AT&T," Deutsche Telekom CEO Rene Obermann said, in a statement.

"Now we need somebody who can convert initiatives into market-successes," he added.

Why this matters
Humm presided over the attempted $39 billion merger between AT&T Inc. (NYSE: T) and T-Mobile last year, which collapsed because of anti-trust concerns. After that abortive union, T-Mobile must now attempt to claw back customers from its larger rivals and chart an upgrade path to faster data services with 4G Long Term Evolution (LTE).

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— Dan Jones, Site Editor, Light Reading Mobile