Deutsche Terrorkom & their prepaid backend online. One way of adding credit / airtime is to have them debit the amount from your bank account. But in order to use this service, you will have to visit a Terrorkom store and register your bank account with them. Then, when you are ready to recharge the prepaid account, you will still be asked to re-enter your bank data. It takes many steps to do all of this – while the other prepaid competitors on the German market already provide much easier processes.
They also confirm every single step with an extra e-mail/notification and when I ordered something in the past, I was sent a letter with 6 pages.