Ok now after 2 days testing and looking after the problem with all this lovely help and support I can tell you: we´ve made it and it is working.
We managed to get it to the stage to make calls but the problem was to receive calls and to hear the other person on the phone. They did hear me very well. And Sipgate.de
charged normally for every call I made. even if it was interrupted after some seconds.
I thought to give it another go and changed the username from "Administrator" to "admin" with using the 11-digit password starting with CP...and it is working! I made my first call to family and friends in Germany and it was a really good and stable connection.
So the problem was (beneath my really small VOIP configuration knowledge - sorry for this
) the router from Plusnet and the disabled SIP ALG Signal.
Nsne suggested that Sipgate is using some finicky settings but they are advertising that you can use it from all over the world and charging for the calls so I thought if someone is blocking this it should be the Internet Provider (as I experienced it before especially with Plusnet...). so I hope that nsne now can manage as well to receive calls and to hear the other Person on the phone.
Thanks to you all, in particular to WelshPaul who really did a lot to get it work
That was really kind and helpful. Happy!