How to prevent iPhone from auto-connecting to unknown Wi-Fi? One thing I never do is to connect to foreign Wi-Fis. But my iPhone decides to automatically connect to those.



Translated:


  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?
iPhone will only auto-join networks that are set to be joined automatically — it won't connect to any random network. SSIDs can be set to auto-join by being manually connected to in Settings → Wi-Fi in the past (the default is auto-join and you can tap the ⓘ to disable auto-join) or because of a profile installed on the device potentially by your carrier or workplace (accessed through Settings → General → Profiles).

You also might need to clean out the check mark to remember networks if you sync settings over iCloud. When one device remembers a network - it will sync to all devices and computers.