Okay, if you activate the normal speaker function while talking, then the screen proximity sensor doesn't turn the screen off. Have you tried that? Have you tried an app called 'Speaker Switcher'? It automates the activation of the proximity sensor with the speaker phone option and may help. Have you contacted your seller to find out where the phone is from? Unless you bought VERY early, it should still be under warranty somewhere. And, even if you have to pay Nokia care to fix it, it may be less than replacement costs.

Let us know if any of this helps. Sorry your phone dropped.