I suspect that the telephone connection cable item no. 242460 RJ11 to TT89 could work. But you can never be sure because there is a lot of confusion about telephone cables.

I am not aware of any adapters from the German to the Swiss telephone plug. If you have such problems, you usually take the diversions via the international RJ11 plug.

That's what happened to me. I simply used the cable from the old phone, and it worked. In my opinion, it should be declared better...