After reading When Lightning Strikes - by Johannes Ullrich - ISC sans diary I was reminded that I could do a bit more about surge protection. We don't live in Florida, but we have do have lightning storms and they can damage equipment.
The first step was easy: Rerouted a few cables, and now the incoming ISDN line goes through the network/phone/isdn surge protector on the APC back-ups CS 350VA. The surge protector is there, so it is a better idea to use it. I have seen the inside of an ISDN adsl splitter after a lightning strike and that was not a good sight.