This is more for myself than for others, but, if you can use it, go ahead. In The Netherlands here, a fair amount of the decent modems sent out for DSL and fiber-to-the-home connections are Fritz!Box routers. These also have a fairly decent phone system built-in that can be hooked up to ISDN or POTS, but DSL and phone signals both go into the same port (but different pins obviously) on the Fritz!Box for some insane reason. A special splitter cable is used to make this work. It can sometimes be a pain in the ass to get these so I took time to write down the pinout.
RJ-45 connector going to Fritz!Box port
Note: technically this order violates the standards in that blue/white and blue are reversed. Since this pair is used for the DSL signal, though, it doesn't really matter.
RJ-45 connector that goes into DSL splitter
1, 2 and 3. (nothing)
6, 7 and 8. (nothing)
Note: AVM delivers a converter that goes from RJ-45 to RJ-11 for this connection. If you need RJ-11 and don't have the converter, just use an RJ-11 connector and use the inner pairs to connect the blue pair.
RJ-45 connector to ISDN (or POTS via adapter)
Note: the inner 4 pins are used for the ISDN digital signal. The outer pins aren't used on ISDN equipment (but for POTS) so are ignored by ISDN equipment, but with a small adapter that goes from RJ-45 to RJ-11, those outer pins are brought from pins 1 and 8 on the RJ-45 plug through the adapter to the inner pair of the RJ-11 plug. That allows the converter to make the ISDN connector a POTS connector. If you want, you can choose not to use the mentioned RJ-45 connector but instead leave the ISDN pairs out of the picture and just connect the orange pair straight to the inner pins of an RJ-11 connector. Such a plug would then look this this:
(leave the brown and green pairs not connected on this end)