In the past months I’ve tried to get Asterisk working a couple of times, but most of the time I had to give up because of the overwelming number of configuration files. Finally I got everything up and running. Especially this post was helpfull.
Now I can make calls through several providers (xs4all, lowratevoip) based on dialing rules. Incoming calls also are working already and I just filed for the transfer of my old ISDN phoneline number to an xs4all voip account.
One of these days I’ll document my settings here, so it might help somebody somehow.