I have the same problem with my Dutch Lumia 800. I tried every solution that you guys give but nothing seems to work. I have a serious problem because i use my phone also for my work and i can't call anybody otherwise my costumors wil have long voicemails to listen to with al lot of cursing. Please help Nokia!!
I have the same problem, everytime i call someone my screen turned black and freezes. And i have to hold the power button for 10sec. But this happens to me everytime, so how can we solve this?