The problem is the distraction by the call itself!

Instinctively and from own experience I can agree to this. However, when on a call the driver can still look at the road, while starting a call, the driver has to look at the phone, ie move away the eyes from the road. That part of the call is maybe the most dangerous part of the entire call. When the conversation is getting intense, the time our brains can spend on driving the car and look out for dangerous situations on the road is becoming less and we tend to go into 'auto pilot' mode.