isdn does not work, fritz avm (pnp?)

isdnutils complained about missing devices. manually creating the devices does
not work because of udev (MAKEDEV should give a warning!). I tried the method
described at:

http://www.ubuntulinux.org/wiki/IsdnHowto
That means manually installing the packages:
pppdcapiplugin_3.3.0.20040728-2_i386.deb libcapi20-3_3.3.0.20040728-2_i386.deb

This looked good but dialing fails with DISCONNECT_IND reason 0x3303 (Protocol
error layer 3) for plci 0x1h
Strange thing is: The error looks identically if the card is not connected to
the isdn network which should NOT give a layer 3 error.