Contuined Billing on Terminated Contracts

I had a 24 month contract with Vodacom which ended in April/May 2014. I only cancelled the contract in October 2014. I was told that my final bill will be end of November 2014, thereafter I will be on prepaid. I received a final statement in February 2015 for R1.50. In May 2015 I was told that I have not been paying my bill and that all services will be terminated i.e. the phone/ number will not work. Now I get an e-mail from the lawyers saying that I owe over R30,000.00. My account was paid via debit order, which I did not cancel. Vodacom said they will cancel it after my final billing. Which they did as I have not been seeing any payments off my account to Vodacom. If my services were all terminated how do I now get a bill for over 30 grand?