Hope that this would work...... You may amend as per your requirement
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To XYZ,
Date:
Subject: Notice of possible occurrence
Dear Mr. XYZ,
This has been observed that after multiple verbal warnings, we have found that you have been using your mobile phone very frequently and that too very loudly in hospital premises on duty, which is not at all acceptable. It has been told to you many times but not much adherence has been measured.
Please consider this letter as a warning to you that if you are caught again for this act, it will definitely lead the management to take strict action against you.
It is advised that you kindly adhere.
Thanking you
Yours sincerely
ABC
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Regards
SJ