I would bet on an HW issue, maybe something linked to the proximity sensor?
But if this is only happening on a single device after 6 months of usage, or you're piling up a lot of cruft or I don't see what can generate this behaviour.
Can you collect some logs in the field to understand what is going on?
RxLogger is already installed on the device and should be easy to enable it. then you just need to have the customer send you the logs to see if there's something strange.
I'll try to get the user to get the RXlogger working on monday. About the RXLogger: so basicly press play, work with my app and at the end of the day get all the txt files?
It happened with his collegue as well that same day they told me, so on 2 devices. So i am not saying it isnt my program causing this problems, but i never had a reaction like that :-)
thx in advance