I don't think that fault has anything to do with dirt getting into the phone. From my experience it happens when you make a call and cover the sensor and/or lock the phone. You described exactly, the phone gets locked, stucked, frozen...it is REALLY annoying. Talking about Lumia 800, I'll not even comment the cover Nokia put on the charger (mini-USB) connection, it's a magnetic-plastic flappy thing stupid and pointless. Other than that, it's a good phone.