Most people throughout the world drive manuals (and they damnwell should!), so holding a phone in your hand is a bad thing as you cannot change gears. Despite how it's been repetedly and clearly shown that it more than quadriples your chance of having an accident, I'll go ahead and list some of the more obvious problems.
The phone is pressed right up against your ear, meaning you have a degree of noise suppression, not hearing an emergancy vehicles siren is not a safe problem (this is obviously not a problem with hands free, the radio or talking to a passenger).
Text messages: Need I say more?
Digging the phone out of your pocket can often lead to tears, a car isn't designed to have it's driver moving about and funbling with small electronic devices.
People often get important, pressing calls on their phones, as I'm no doubt sure you realize, what you may not realize is that the majority of people focus on the phone when they speak on it (they do not do this when speaking to passengers or if using a hands free kit), this is because they are physicially holding it and it is very apparent that they are using it.
It also removes a large amount of stress from the driver if they aren't in a position where there boss could call up and abuse them, there's always that moment when your phone rings and you don't know who's calling or why, if your life is full of drama (as most are at some point) it will incite a level of panic, thus reducing the drivers control of the vehicle.