I have found a document which describes how to troubleshoot ISDN problems with the command:
According to this document, one could determine wether the problem would be internal or external to the router. This should be done by analyzing the values of "Write_sid" (shown as "wrote=x" on most routers) and the SID interrupt status "reg=x".
However none of these values are displayed after typing the "debug bri" command.
Does anybody have experience with this?