Dear Brindha,
If employees get time to talk on mobile or chat then it appears that they some amount of disposable time with them. Therefore, rather than chasing them for not talking on mobile phone, you may do the workload analysis and find out whether they have sufficient work.
In your company you face a challenge of people talking excessively on mobile. In few companies opposite happens. People of few companies are (in)famous for not attending or returning the calls.
Secondly, do top management officials prefer "phone" as media of communication? Do top boss has habit of picking up phone and start talking? Is he reluctant to write mail? In modern workplace, one should use all type of media of communication i.e. e-mail, SMS, telephone, WhatsApp message and so on. When to use what media of communication individual's discretion. However, lopsided emphasis to any one of the media obviously impacts the work. In your company's case it is telephonic communication.
Thirdly, do employees work beyond their specified working hours? Is working late is a custom in your company? Do top level official like people to work late in the office? In that case, make a rule that all employees must leave their workplace exactly in time. For working extra, they should take permission.
Fourthly, about socialisation. While socialisation at the workplace is important, has your company promoted culture of over-socialisation?
Lastly, about concept organisational communication. Possibly, your company has given too much importance to the personal communication. Now personal communication has started overriding the organisational communication.
In view of the above, I recommend you having a look at the organisation culture of the company. Discuss the issue with the managers. Find out what are their problems. Talking on phone could be a job requirement also. For example, average Purchase Manager talks on the phone for at least 4-6 hours per day. But then this is what the nature of his job is. Therefore, find out who needs exemption.
Average Indian is garrulous. We are not impersonal like British. To certain extent you could be challenging the DNA itself!
Thanks,
Dinesh Divekar