I think flyingfkb's explanation might explain why I lost credit. I know some apps can use internet AS SOON AS I connect to internet. I am aware of it, so I always turn off the mobile data before plugging in the sim card abroad. Anyway, I lost something like 10e, I hope they get the credit back. Unfortunately, I left the country, so I cannot check or talk to the CS. I am a bit disappointed by the german tech guys as I always thought Germans are very reliable...