If you are having HR Manual include it as policy "Employee Cell Phone Policy".
First of all ensure that you have proof for usage of mobile during working hours. You may pass a circular advising all employees not to use/reduce mobile for personal work during office hours. Still if employees continue to use mobile ( calls/ accessing social web sites) during office hours then you may ban using mobile during office hours. You may use the following draft.
We are aware that mobile phones have become an essential part of everyday life for all of us. If used correctly this technology advancement will be very beneficial. But of late we observe that many employees are using it impudently by accessing social websites, making personal calls, for chat etc during office hours. This creates problem in the work place which primarily distract employees from work. Hence it has been decided not to use mobile phone during office hours. All employees are hereby requested to hand over their mobiles to the security staff which will be handed over back to you after your shift hours. However all are permitted to use office phone extension number for any emergency call either to call or receive.
We request all employees to perceive this measure positively.
For any clarifications please contact HR Dept."
28th May 2019 From India, Madras
30th May 2019 From India, Mumbai