Dear friend,
Your post is too short. It does not give enough information. What is you mean by "casual behaviour"? Can you please give example? Secondly, what exactly happens when employees use their mobile for the personal work? Is talking on mobile hampering their work?
As far as the casual behaviour is concerned, you need to make a small manual on workplace etiquette or workplace behaviour. In this manual, include topics like method of communication, protocol of communication, rules of written communication, rules for using the mobile phone etc. This manual should be part of the induction training itself.
If the employees are talking too much on mobile phone then possibly they do it because they have time to do it. Therefore, what is required is to find out whether they have surplus time at their disposal and how to fill that time. To do this, you need to do workload analysis. Secondly, employees could be talking excess because managerial control may be missing.
Sending emails or sending office circulars does not work in the long run. After issue of email or office circular, there could be change but it may not last long. After a month or so, there could be relapse.
Thanks,
Dinesh Divekar