Unfortunately this is not true. When you are conversing with someone in the car, that person is often aware of the situation you are in and will either stop speaking, or more importantly, provide you with the "quiet" time you need to rectify or maneuver instead of expecting your immediate response. This is also usually true of other things drivers do, such as tuning the radio or sipping a coffee. They "usually" wait until "they" know they have the ability to do it safely.
When you are on the phone, the person on the other end has no idea what your situation is, and will continue talking and asking questions during times when the driver needs to focus. Sure, many can simply indicate they need a moment, or ask later for someone to repeat something, but unfortunately, many, especially those who live on their phone, dont want to interrupt, or feel that they have to... and bang.
Grant