Dear Anita,
How you came to know that the employee has been unauthorizedly absent for 15 days?
Are you his functional head?
If not so, did you receive any complaint from his functional head?
Whatever it is, first, send a letter to his last known address by registered post directing him to report for duty within three days with his explanation in writing for his unauthorized absence. Also mention in the letter that any failure on his part in this regard would entail disciplinary action against him under the relevant rules of the organization.
If the letter is unserved or there is no response from the employee, frame charges, conduct a domestic enquiry ( in case of his failure to attend the enquiry, the enquiry officer would set him ex parte), after analysing the findings of the enquiry report, issue a second show cause notice about the proposed punishment and then finally award the punishment of dismissal by means of a speaking orders. If the absconding employee remains incommunicado all along, effect service of communication at every stage by means of paper publication particularly in vernacular news papers.