One Number is an Android application developed by Deutsche Telekom AG, available for free. This app is designed for users who want to keep their mobile and landline numbers separate while on the go. The app is only available for users who have the Telekom mobile option "One Number."

One Number allows users to decide which Telekom phone number will be displayed when making a call. Users can choose to display their office number or the number of their firm's headquarters. This feature is useful for business owners who want their partners to know which company is calling. The app also allows users to select specific contacts to whom their mobile number will be displayed. This is useful for friends and family members who may need to call the user's mobile number directly.

Overall, One Number is a great app for users who want to keep their mobile and landline numbers separate. The app is easy to use and provides a great way to manage your phone numbers. We hope that Deutsche Telekom AG will continue to improve the app and make it even more user-friendly.