I stopped all my van drivers using mobile phones whilst driving some time ago, it is now viewed as gross misconduct if they do.
I eventually got it through to them that it wasn't a matter of the £30 fine, should you injure or, heaven forbid, kill someone on the road whilst using a phone, the chances are that you would not just be banned from driving for a considerable time, but could also face a prison sentence for causing death by dangerous driving or even manslaughter. It is quite easy to check if you were using the phone at the time of the accident.
This seems to have worked