Contract paid up but still being invoiced.

I paid all 24 installments of the contract as per the 24-month contract yet I was billed for another month and then sent to debt collectors. I have spoken to Nandi & Seipati from Retentions and they have counted all payments and confirmed that all the 24 payments were received however each time they escalate - they are told I am still liable without providing a reason WHY? No where in the contract does it state that I have to pay an additional 25th payment to cancel the contract. The contract was cancelled in time and all payments were made however I have been handed over. I am disgusted and shocked how this company is still in business because there is not a single good review ANYWHERE. I am tired of speaking to your call centre agents because they cannot provide me with a reason WHY I am paying a 25th installment to a 24month contract.