Windows Phone customers on O2 Germany have had the option to pay for Windows Phone Store purchases with a credit card or through their PayPal account. But direct billing to their wireless account wasn't possible.
O2 will now let customers bill their app and game purchases from the Windows Phone Store to their O2 wireless account. When purchasing an item from the Windows Phone Store, just choose the "Add or switch payment methods" option before confirming the purchase and you should see the option to bill your O2 account.
Source: wparea; Thanks, Olaf and WPfreak8, for the tip!