Difficulty in cancelling cellphone contract

Despite several cancellation requests having been made since November 2015, Altech Autopage Cellular Pty Ltd insists on ongoing subscriptions being paid to them each month. My cell phone has been barred by them each time because they insist on the payment of an arrears subscription being paid in full - yet it has been fully paid to their account with proof of payment supp****. At present they've indicated that my service will be suspended by 14/2/2016, if the current monthly subscription is not paid by then. They have indicated that any available airtime can be used, but they haven't unbarred my phone to enable me to do so despite several requests. The contract is in the name of my mother, Cynthia N Barrett, who is 99 years old and totally incapacitated. I have a General Power of Attorney to manage her affairs. Altech Autopage's attitude of total indifference is very disappointing. I am concerned that they will continue to bill me for a service no longer needed. All my attempts to resolve this have produced nothing more than my being sent confusing \No Reply"electronic messages that don't help to resolve the situation. Please help me to do so. """