Billing issues

Account no*** EFT Ref No ***. I've been paying telkom mobile account manually for months on time based on the amount that is sent via SMS as the invoice that comes out is always wrong. They can't seem to update my banking details on their system when I call them to update me. I paid R3372.99 at the end of October which was the amount due. Then I get an invoice on 3 November showing that the amount was paid and the next amount due is R2606.25 due 30 Nov. Now my account is in arrears with this R2606.25 which means I must have paid R5979.24 in total for my account to be up to date. This is completely insane. When I call the call centre they keep telling their system does a credit with my manual payment and a debit with amount that did not go off my old bank account. This makes no sense and now they want to suspend my account!!