Good day, info-inyo-living.
I can share some information regarding using QuickBooks Online with a German phone number.
If you're using the QuickBooks Online US version, then you need to use and enter a valid US phone number.
To use a German phone number, you can subscribe to QuickBooks Online for Germany. You might want to visit this link and choose your country: https://quickbooks.intuit.com/choose-country/.
I recommend visiting the links below. These will provide you with resources and videos about managing your QuickBooks Online account, income, expenses, inventory, and other business information:
Video Tutorials
Resource Hub
You can always get back to me if you have any other QuickBooks concerns. I'm here to help. Have a nice day!