Sunday evening I finally had time to look at the new home server greenblatt and I tried to get the sitecom dc-105 isdn card in NT (network termination) mode connected to the fixed line (outside) isdn port of the fritz!box 7170. It took a bit of work as a lot of documentation about the mISDN drivers mentions NT mode but the needed cable isn't very well documented. I finally found it, chapter 2.2 of the PBX4Linux manual. By itself the crossed cable did not work (and the fritz!box is good at diagnosing problems with SIP dialing, but just goes 'meh' when ISDN dialing fails). I didn't need the fancy solution with power, but I had to look for a while for termination resistors. I remembered the sitecom dc-105 isdn card had some jumpers near the ISDN port. Those are indeed 100 ohm ISDN termination resistors. Nowhere to be found in any of the manuals of the dc-105 online.
After setting those jumpers it all started working. At first the dialtone sounded weird but that was caused by[general] country=usin indications.conf. Changing this to[general] country=nlmade it suddenly sound a lot more familiar: KPN style. Now the test calls are running again via the modem connected to the fritz!box.
Conclusion: the jumpers on the sitecom dc-105 are isdn termination jumpers and can help to make an NT mode cross cable work.