Weird overlay

Hey guys,

When I activate the simcard in my ipad mini retina, I sometimes get an overlay that blocks my entire screen. It is almost entirely opaque, and say "your phone number: xyz" in the center, with xyz being the number of the simcard's phone account I guess. Interestingly the text is in German (I live in Germany) even though my iOS is set to English.

The annoying part is that there is NO WAY to dismiss this overlay. Not even doube-tapping the home button works; I have to reboot the device.

The phone carrier claims it must be an iOS configuration, but of course I seriously doubt it. My vague suspicion is that this is related a a service SMS of sorts, but I have no hard evidence for it one way or another. Never seen anything like this on my iPhone.

I unfortunately have no screenshot of this, I will try to take one next time this happens, but perhaps someone has an idea what this might be?