NEW YORK (CNN/Money) - Deutsche Telekom AG tapped Helmut Sihler, a member of the company's supervisory board, to temporarily succeed CEO Ron Sommer, who resigned Tuesday.

Gerd Tenzer, head of Deutsche Telekom's technology and networks division, had been thought to succeed Sommer. Instead, he was named deputy chairman. Sihler will serve for a period of six months, Deutsche Telekom said in a statement.

In a statement Tuesday, Sommer said he resigned because Deutsche Telekom's supervisory board no longer had complete confidence in him and his strategy.

Sommer, 53, transformed Deutsche Telekom from a state monopoly into a modern technology giant. Sommer fell out of favor with investors due to a costly expansion which led to DT's $67.9 billion debts.

"These are circumstances under which a cooperative collaboration between the Supervisory Board and myself is no longer possible," Sommer said Wednesday.

In a press conference, Sihler, vice chairman of the board at Novartis AG, said he will focus on finding a successor and selling assets to reduce the company's heavy debt load, the Wall Street Journal reported.

The Bonn, Germany-based Deutsche Telekom, which had revenue of $47.9 billion last year, is one-fifth owned by U.S. shareholders and its stock has dropped 90 percent since its high two years ago. U.S. shares of Deutsche Telekom (DT: Research, Estimates) gained more than 1 percent to finish at $11.06 Tuesday.