Five years on from the mobile phone law was introduced and drivers are still dying 'needlessly', according to the Royal Society for the Prevention of Accidents.
But it claims fleet managers can help tackle the problem by urging their drivers to switch off their mobile phone, whether hand-held or hands-free.
The new law making it illegal to use a hand-held device while driving was introduced on December 1, 2003.
But RoSPA claims that casualty figures show that people are still being needlessly killed in phone-related crashes.
"It is disappointing that people are still being killed and injured on our roads because telephone calls or text messages are deemed more important than someone's life," said Kevin Clinton, head of road safety at RoSPA.
"Our advice to drivers is clear: switch off your phone when you get behind the wheel and let voicemail do its job, and we urge employers to make this part of road risk policy."
Observational research has revealed a reduction in the number of motorists using mobile phones since 2003, including a fall after harsher penalties for breaking the law were introduced last year.
However, there were 25 fatal accidents, 64 serious accidents and 259 slight accidents on Britain's roads last year involving a driver using a mobile phone.
Meanwhile, the University of Warwick has revealed that using a hands-free device can add five metres to a driver’s braking distance.
“Our research shows that simply using phones hands free is not enough to eliminate significant impacts on a driver’s visual attention,” explained Dr Melina Kunar from the University of Warwick.
Mr Clinton added: "While the law specifically covers hand-held mobiles, research such as this means it is wrong to suggest that using a hand-free device is safe.
“Police are able to check telephone records when gathering evidence in careless driving and dangerous driving cases and use them to show someone was distracted.
"This can lead to tougher sentences, because using a mobile phone of any kind while driving is likely to be viewed as an 'aggravating circumstance'."