Then his boss is stupid too. Does he really think spending money on a solicitor will get it over turned ? Your son was bang to rights so what is he going to use to suggest he isn't guilty ?
I'd bet a HUGE amount of money that they don't expect their employees to break the law and therefore don't expect them to answer their phone while driving. I'd bet an even bigger amount that this is detailed in the staff handbook.
My company covers it very easily - if you answer or use your phone whilst driving on company business ( or anytime in a company vehicle ) and they find out you get a written warning. Guess what - nobody touches their phone while driving.
What if it's someone from his company who does hve access to the tracking application and is using a desk phone to ring him ? How do they send a text ?