T-Mobile USA CEO Robert Dotson says he is leaving the company after 15 years in service to "transition to new opportunities."

Dotson's departure comes as the Deutsche Telekom subsidiary struggles to compete with its larger rivals in the increasingly saturated U.S. market.

Dotson will remain T-Mobile's CEO until February 2011, when he will be succeeded by Philipp Humm, who was CEO of T-Mobile Germany from 2005 to 2008.

Deutsche Telekom CEO Rene Obermann said he "sincerely regret(s)" Dotson's decision to leave but was "grateful" that Dotson would be staying on for another year to ensure a successful handoff to Humm.

Dotson will remain on as a T-Mobile USA non-executive board member until May 2011 after Humm takes over his position in February.

Obermann credited Humm with the successful turnaround of Deutsche Telekom's German operations and the market introduction of the iPhone there in 2007.

"[Humm] took over a company in a declining market and successfully turned it around during his tenure to become Germany's leading mobile operator in both subscribers, as well as service revenues," Obermann said.

Humm will face a similar task when he takes the reigns at T-Mobile USA. The carrier was once a growth engine for its German parent but has struggled in the past year with declining sales and sluggish subscriber growth.

T-Mobile USA lost 77,000 customers in the first quarter after defections from its contract customers failed to offset the addition of new prepaid subscribers. Sales slipped to $5.27 billion in the first quarter from $5.39 billion last year, and ARPU followed suit, slipping four percent to $46.

T-Mobile is deploying HSPA+ and has cut the prices on its prepaid service in a bid to attract customers but lacks the spectrum to deploy 4G, sparking rumors that T-Mobile will partner with a third party to deploy fourth-generation services.

The company's troubles have prompted Deutsche Telekom's shareholders to push the company to divest its U.S. assets. Obermann has said Deutsche Telekom is committed to its U.S. subsidiary and has no immediate plans to sell the company.

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