I was having a similar issue with my phone, but then I was using a case that snaps over the back of the phone (it doesn't go over any of the sensors...
I was a little annoyed because I've also stood in the middle of a supermarket trying to end a call and not being able to, only getting a glimpse of the keypad screen when I press the lock button.. I've tried to call my mum a couple times and ended up leaving
a frustrated voicemail of me trying to end the call :-/ and I've even had to switch my phone off just to end a call..
However after removing the case, it works perfectly fine again....
I have no doubt it's likely the case that caused the problem, but I don't want my phone to get scratched to hell just so I can end calls when I need to...
Nokia, please sort this out :smileysad: