Edit 20260617: intenral -> internal in the post title
Somehow WordPress.com failed to post it on the scheduled date 20150331, not sure why.
Though ISDN has been phased out in The Netherlands as and almost phased out in Germany, you can still use end user Terminal Equipment on the internal S-bus.
Since scheduling the post for 2015, the same information as the original diagram below appeared in text form on Wikipedia and there it also adds TAE connector information: Fritz!Box: Belegung der kombinierten DSL-Telefon-Buchse – Wikipedia
The Wiert Corner - irregular stream of stuff 