Dear,
If you observed that 'All the Employees' are using the mobile phone and not focusing on work, its better to issue a formal notice to all instead of sending warning letter.
If you feel that after sending the official notice there is no improvement in the staff then you can have a meeting and convey your message by using innovative ideas.
You can have a meeting with managers to check the available work status. Because it seems like employees don't have work to do so they are using their mobiles.
Ask the managers to designate work to employees with deadline so that employees remain busy with work.
After all this if you still find any employee not obeying the rules, you can send him/her the warning letter.