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.

Of what accuracy are these ISDN time stamps? second? millisecond?
microsecond? If it's only good to the second or tenth of a second, it's
likely that the system will indeed be able to measure some jitter in the
signal, independent of how clean the telephone company (DBP Telekom in this
case, nu?) claims it is. Nothing's perfect.

	Erik