Thank you all for participating in this discussion.
I was quite astonished about the traffic this little question
generated.

I had several enlightenments.

As Erik (and others) said, the obvious trick is:
> I'd turn it into an NTP time source in the driver that the NTP daemon can
> read from, but that also returns the microtime() value when the last ISDN
> timestamp came in.

That's what I'm going to do on my home machine.

On the other hand, this timesource is not as excact as I thought.
"Exact within a few seconds" is far enough in my little world (and far
better than the PC clock which is of by several minutes sometimes).
So I'll make a real small solution inside the kernel as well: call adjtime
if timestamp and kernel time are off by more than X seconds, and only do
this once in time Y. This whole feature, as well as X and Y, would be 
kernel config options. I'll use this on the dedicated router system.
The only reason to have a kind of "real" time there is charging prediction
when you don't have subscribed to the AOCD service. And its better to have
the same time as your local switch than to have "the right" time for
this application ;-)

Thank you all,


Martin