The app, which encrypts voice or text exchanges between two devices using a unique code, will be unveiled in mid-March, the Guardian reported Monday.
It will launch first for Android phones and then later for iOS devices, although Deutsche Telekom has not confirmed an exact date for its release.
The service will be run by the company's T-Systems in co-operation with Germany's Sichere Mobile Kommunikation mbH (GSMK), a provider of encrypted phone services and devices.
"To the best of my knowledge, this marks the first time that a major network operator throws its full weight behind end-to-end mobile voice encryption," GSMK's chief executive, Bjoern Rupp, said.
"This is not just in the form of a specialized niche product, but in the form of a mass-market-compatible product that will be rolled out to all of its customers," he said Monday at the Mobile World Congress phone industry conference in Barcelona, Spain.