One thing I never do is to connect to foreign Wi-Fis. But my iPhone decides to automatically connect to those.
Welcome to the Telekom HotSpot (german network provider). You are connected to the internet via your SIM over Wi-Fi and are using your HotSpot flatrate. There will be no additional cost.
Unfortunately I've seen the message half an hour later so I assume I already sent data via a foreign Wi-Fi in the background. How do I stop the phone from connecting to new, for me unknown wifis, without turning Wi-Fi off completly? Is this a general 'feature' or only in branded iPhones?