Apparently in your case you have caused abrupt termination and as per internal policy for HR the HR is not issuing relieving letter.
Relieving letter is issued post all settlements. It signifies that nothing is pending and employee has been properly relieved.
As pointed out above as per Model Standing Orders 'Service Certificate' should be issued to all employees.
In your case the establishment has collected the notice pay.
Did you offer to handover the charge? Were any tasks/assignments left pending?
If you were on leave before resignation then apparently no tasks were pending.
If you have handed over the charge/company property and no tasks were pending then there should be nothing pending to be done by you.
If officials of the company are not responding then you may submit representation to appointing authority, MD preferably by redg. post.
Later you can approach employees unions, Trade Unions, O/o Labor commissioner, Inspector under (Name of your state) Shops and Commercial Establishments Act, If it is commercial establishment, Court of law...............
