unfair practice on a contract

I had a contract with Vodacom, then I lost my job, my contract expired then we went on a month to month arrangement, few months down the line I could not afford to pay the agreed amount, so now they took me to the lawyers saying I must pay for the next 13 months until the contract expired in June 2020 which in not a valid contract as I know that we on a month to month contract. they said I should have written a letter to cancel the contract, no one ha sever advised me of the since they started communicating with me about my overdue account. in April I wrote a cancellation letter, they said its not acceptable as they have handed over my account in March the 16.

Vodacom if robbing people. a month to month contract should be cancelled if you send the cancellation fee and their customer services people are not helpful.