The DSL modem will *probably* work. Germany has a different voltage on the telephone line, but most will cope with it.
However, trying to get hold of the correct cables and a microfilter (DSL Splitter in germany) is a nightmare. Most places won't sell them - insist you go to
T-com, who will only sell you one if you are an existing customer. Took me two weeks to find one, only to find I needed yet another special cable. Finally got the cable to find the phone won't work with Alice unless it's connected directly through their DSL modem.
In conclusion, yes it'll probably work, but it's not worth the hassle - stick with whatever your ISP supply you and let them take care of the problems.
Mat