At the BWV and the BCV, (where my most recent stays have been) I've just brought along another phone line and plugged my laptop line into the data port on the side of the telephone.
It's been a few years since I stayed at OKW, but I believe their phones are the same, (with the data port on the side.) One day, not realizing there was a data port connection on the phone, my DD just had unplugged the phone line from the phone in the living room and plugged it directly into her laptop... worked just fine.