I'm always in dual, and it was getting 3G when it got cut off as I made that call.

I'm nowhere near a border, and while my operator (Free Mobile) is using another's (Orange) infrastructure, there is no switch between them: it's always Free.

What is quite odd is that I made that call at home: before the call everything was fine and I could connect to internet to my heart's content, and right after I "recovered" from the call: data roaming all over.