If you do not want to enter the dial-in-number every time you make a call, there are different ways to avoid this. For one thing you can assign DNumbers to frequently called contacts. You can save the DNumbers in your phone book, so that you can call the contact directly. In case you are a Fritz!Box user, you can configure it with only one click. Afterwards, all international calls will be supported by DCalling, without you having to use the Dial-in-number every time. You can have a look at the instructions here.