Actually i have the same problem but i didnt dropped it or anything !
as i mentioned before my N900 sleeps on my bed and im sleeping on the cold floor ( just clearing how much i take care of it )

anyway when i receive a new call sometimes my N900 get stock and it shows "not charging" notification, and i only hear sounds just like when you want to make a dial up connection on the PC ( the alien one )
so i need to jack in headphones to be able to make calls

THE END

Thanks in advance awaiting for replys too

./sifo