It's the way it's designed. It's been discussed before. It's a safety feature apparently. If you are talking on the phone and the call gets disconnected, then your phone rings you may harm your ear with it being so close. Or that's the Sony explanation
I explained to a Sony rep on here that my phone makes a "beep" when a call is disconnected thus telling me to withdraw the phone from my ear. But apparently they think we are all morons that don't know our **bleep** from our elbow