There are plenty of work sites and offices where using a mobile phone during work hours is a sack able offence.
I fear you seem to miss the points relating to Contractual Clauses that prohibit their use. In the case in point - there appears to be nothing written contractually, nor any formally signed documentation between the employee /employer to make the use of mobile devices during working hours a dismissable act, therefore your case would more than likely be quoshed in court, because the Employer has not ensured the employee is aware of the company policies.
Statutory contracts do not usually cover every eventuality, so one should always cover as many bases as you can think of when employing others to safeguard themselves.
Ignorance when it comes to employers is heavily slanted in the employees favour unfortunately - as the law is such that people can go to work and act like they have nothing between the ears and do really stupid things!