"The available research indicates that cell phone use while driving, whether it is a hands-free or hand-held device, degrades a driver’s performance. The driver is more likely to miss key visual and audio cues needed to avoid a crash. Hand-held devices may be slightly worse, but hands-free devices are not risk-free."

Though it doesn't link to any studies and I'm too lazy to go searching, so if you want a primary source you'll have to find it yourself...

https://one.nhtsa.gov/Driving-Safety/Distracted/Policy-State...