BSNL — How to cancel a contract,which signed for 16 year with BSNL

Address:

Srikakulam, Andhra Pradesh

BSNL tower was installed in our place last 6 years ago. my father had signed for 16 years contract. they are paying Rs500 per month.However they are not paying regularly and they have not given any documents yet. We would like to cancel the contract. Can anyone suggest us how to approach to resolve this problem