Today a mechanic came to check out the line for my DSL connection. Apparently XS4ALL had my DSL patched through on my second phone line. The second phone line was never properly set up; the mechanic told me the entire line was missing at the distribution point. After some rerouting the DSL line came up.
Unfortunately only 14mbps down and 1mbps up (not the 16mbps down and 2mbps up the subscription offers at maximum) but that might be limited by the quality of the phone line. I can only hope at some point they will upgrade so that higher speeds are attainable.
Anyway, I am no longer limited to my iPhone for internet. For that, I am very happy.