account expired but they are still calling me

I had a contract with Vodacom on cell: *** for an Iphone. On the 2013/04 the contract was ending. I have already called in but the debit order didn't go through for that month so I was advised and paid on line. but after 2014 till now they keep calling me that I am in errors and will be backlisted. I keep explain to the different call agents and forward them the internet page but I haven't been successful. please help and make this go away. they cant even explain what do I owe