After adding surge protection to the ISDN line coming into the asterisk server I went out and got the Trust surge guard PW-3500.
Note the This product is no longer available in the current Trust assortment on the Trust website. I bought it at mycom: Trust UPS & Overspanningsbeveiliging Surge PW-3500 - MyCom.nl and I noticed on the outside of the package that it does not mention whether it can be used with an ADSL modem, it just mentions phones and fax machines. A note about suppressing high-frequency interference made me wonder whether it might filter high-frequency signals on the phone line a bit too much, killing ADSL throughput. The salesperson at MyCom could not tell either. But MyCom has customer friendly policies for returning articles which aren't what you expected. Even "I could not make it run with Linux" is a valid reason for returning it. So I took it home, and first thing I noticed in the manual inside the package is that it states that this surge protector is also designed for ADSL. They could have mentioned that on the outside.So now it is living in the phone line (and power supply) for the ADSL line.