Dear Brindha,
If employees get time to talk on mobile or chat, then it appears that they have some amount of disposable time with them. Therefore, rather than chasing them for not talking on the 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 a few companies, the opposite happens. People in some companies are (in)famous for not attending or returning calls.
Secondly, do top management officials prefer "phone" as a medium of communication? Does the top boss have the habit of picking up the phone and starting to talk? Is he reluctant to write emails? In the modern workplace, one should use all types of media for communication, i.e., email, SMS, telephone, WhatsApp messages, and so on. When to use what medium of communication is an individual's discretion. However, a lopsided emphasis on 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 a custom in your company? Do top-level officials like people to work late in the office? In that case, make a rule that all employees must leave their workplace exactly on time. For working extra, they should take permission.
Fourthly, about socialization. While socialization at the workplace is important, has your company promoted a culture of over-socialization?
Lastly, about the concept of organizational communication. Possibly, your company has given too much importance to personal communication. Now personal communication has started overriding organizational communication.
In view of the above, I recommend you take a look at the organizational culture of the company. Discuss the issue with the managers. Find out what their problems are. Talking on the phone could be a job requirement also. For example, an 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 an exemption.
Average Indians are garrulous. We are not impersonal like the British. To a certain extent, you could be challenging the DNA itself!
Thanks,
Dinesh Divekar