The probe was launched after US authorities asked German officials to aid their own inquiry, which also includes suspects who are not employed by Deutsche Telekom, the company said in a statement released late on Tuesday.
Allegations include links to a meeting with the head of Makedonski Telekom (MakTel) in 2005, during which the company was paid a dividend on the condition that the Macedonian telecommunications market remain closed to other competitors.
This has also been connected to other bribery payments by a third party, the company said.
Obermann denies any wrongdoing, and Deutsche Telekom, Europe's largest telecommunications company, said it is cooperating with investigators.
Obermann also said corruption is not tolerated within the company and was not under suspicion of wrongdoing while already working with US authorities for the last four years.