It's all about attention.
It's not about the mechanism (the phone). Don't let anybody kid you.
Speakerphone, headset, in-car-audio, earpiece... doesn't matter.
Sure, most of us can multitask and many times we do so just fine without a problem. It's a bit of the "struck by lightning" philosophy that if we haven't been struck it can't happen to us. Sadly, we understand more about not going out during a thunderstorm holding a six iron golf club than we do about driving and talking on the phone but more people have car accidents each day than get smoked by a bolt of lightning.
Driving *IS* multitasking. It's extreme multitasking when you think about it and that doesn't even include kids in the back, a chatty passenger, or bobbing your head while listening to the radio.
Ever watch someone at home when they're on the phone? Ever see their eyes stare at the lamp in corner or catch them meander aimlessly around the house looking at the floor as they talk? This is what happens when attention gets fixed. Fixed attention is hard to stave off when driving. Sometimes we can get away with it more and longer... but eventually the conversation may engage you enough that your attention goes from being passive to active and it's at that moment it gets fixed - fixed on visualizing the person you're talking to, fixed on the scenario they're talking about, fixed on the argument you're having with them.
It's at this moment you look up a bit and realize you've covered three city blocks with no real memory of passing anything. You don't know what the temperature is on the bank sign like you normally do. You don't know what movie is playing even though you normally glance at the marquee outside the theater. You didn't even realize what your speed was until you purposely looked down nor notice the vehicles behind you - which is a normal thing to do when periodically glancing in the rear-view mirror.
Essential checks and balances, necessary patterns of checking mirrors, speedometer, distance, speed signs, *other* drivers busy doing their thing... that's where attention should be fixed.
At least a chatty passenger in the car sees roughly what you're seeing... they can pause their discussion intuitively while you drive. They can see the construction or school zone coming up ahead just as you can. They feel the car slow down and ease off the chit chat while they allow you to concentrate a bit more. It's natural. Someone on the phone can't provide the same, instinctual courtesy.
The radio and music is passive. It might be entertaining but no matter how much you love Rush or Michael Reagen... it's not actively engaging your senses the same was as a phone conversation.
I've taken one mile walks around our neighborhood for fresh air and exercise while conducting business on the phone and I'll be damned if I can remember any of the beautiful day, blooming flowers, cute puppy, giggling kids, friendly neighbors I passed along the way? It's like I get off the phone call and suddenly begin to feel like I'm coming out of a trance I'd been in for ten minutes.
I'm old enough to remember rotary dial phones but also young enough to be rather hip to technology (it's part and parcel of my daily and work life). I remember how thrilling it was to get mobile phones with all the promise they had to offer. I even own an iPhone plus a rather expensive bluetooth headset. That headset is so I can type and talk or walk and talk (when you're self employed you're always working even when you're not). I don't drive and talk.
I could easily indulge in the technology I've grown up with and max it out behind the wheel - I'm certainly set up for it and have an affinity for all things that blink and beep.
But I'd never forgive myself if even a $100, 15mph fender-bender snapped a child's neck back and forth in their car seat no matter how superior my full-coverage auto insurance is. Some catastrophes can't compare with whatever seemed so important seconds earlier and once those bumpers collide you can't take it back.
There's a reason we're seeing more reports from reputable studies demonstrating talking *at all* while driving is dangerous beyond drunk driving.
Responsibility as a driver doesn't mean you don't drive 120mph through a school zone. It doesn't mean you don't do cookies in the parking lot of the daycare. Those are given. Responsible driving means you care enough to 'get it' that some things are just not worth it and you sacrifice a little convenience so you (or someone else) doesn't have to sacrifice a lot of their life.
Go play golf during a thunderstorm if you want. Wear your bluetooth though. You may need an easy hands-free way to call 911.