The human brain cannot handle two thinking tasks at the same time, such as driving and talking on the phone. Your brain toggles quickly between these two
tasks. When driving, this can slow reaction time and cause crashes. ( National Safety Council)

Driving while engaging in activities such as talking or texting on the phone, eating or drinking, reading can distract the driver's attention away from the road and compromise the safety of the driver, passengers, pedestrians, and others.