The law seems to be a bit 'grey'.
If you are stuck on a m'way, engine off and not having moved for an hour it seems you are still 'driving' for the purposes of the mobile phone use. In fact you may not even touch it. One might wonder what happens if you get out and stand with lots of other drivers on the hard shoulder and take your phone with you. The offence would be that you were in the car when you actually touched it to pick it up and get out of the car. What if you opened the door, leaned in and picked up the phone, or if a passenger picked up the phone and handed it to you when you were already out of the car.
However, if you are an HGV driver, whilst stuck in the same jam you can deduct the time spent stationery from your permissible driving hours, so you are not legally driving neither are you parked apparently. This mean a truck driver claiming this as 'dead time' for driving hours is then not legally driving and can legally use his mobile phone (or re-programme his sat nav).
Of course using a hand-held mobile should be illegal whilst driving, but the definitions of what 'driving' actually comprises seems a bit vague or badly defined it would seem. There are direct contradictions as to what constitutes driving.