The biggest risk of talking on a mobile phone while driving is that a phone call is not automatically adapted to the prevailing traffic situation. When a driver talks to a passenger, however, it is.
If a complicated traffic situation arises during the conversation, the driver can delay an answer and focus on the driving without the passenger pushing for quick answers.
In February 1, 2018, it became prohibited to drive with a mobile phone in your hand. You are still permitted to talk on the phone if you use a hands-free device, but for safety reasons this should also be avoided as the phone call itself reduces your attention and concentration.