Earlier quoted context omitted.
Telefonica run O2 in the UK, which is a decent mobile network (and the only one which covers my house).
O2 in Germany sucks, though. You actually have to snail-mail them your resignation letter, there are frequent brief outages in the night and they are often slow at setting up your service. I mean, they do deliver decent internet, but their customer service is awful.
So? You have to send written notice to cancel any contract in Germany. This is totally standard.