Every now and then - its pretty random, but at least a few times an hour - a black screen pops up (see attached image) with 'cancel' 'accept' buttons, and a small back button in the top left... its not a call, it doesn't vibrate, and it pops up ontop of whatever your doing. I have noticed that the phone does talk to the network/cell tower when it happens, as i can here the interference on the desktop speakers.. Have searched / googled, but couldn't find anything so am asking here.
here's the pic
thanks
s.