It boils down to the task being performed and what the brain can handle. Apparently the brain stumbles hard on the task of voice-based texting, even over holding a phone in the hand.
Using test subjects in their 20s and 30s, researchers conducted three layers of tests, in a lab, then in a simulator, then in a residential area on real roads. The device used was an unspecified Samsung smartphone with a hands-free device in a vehicle that was not equipped with an integrated infotainment system.
All the participants had good driving records and the researchers found that the task of hands-free texting caused a “large” amount of distraction. Holding a cell phone and carrying on a conversation posed only a “moderate” distraction. That is in line with a previous study that AAA performed where drivers navigated a course under varying degrees of distraction.
More and more states in the US are making texting while driving illegal and while there is pressure on automakers to limit some infotainment system functions in their vehicles, they counter that would only result in drivers using their devices in-hand again.