Account query

My son moved to Australia end Jan 2020. He went to Vodacom in Rustenburg to close his account and paid the balance outstanding on the contract. Then on 05/05/2020 he received a final notification of an additional amount that was still due, which he paid VIA EFT directly into the account of Vodacom.
Then Vodacom still hand him over to Debt Collectors, being Ncube Inc attorneys. On 21/03/2022 Ncube send him a mail requesting payment (which was done in 2020 already). I send proof of this payment to Ncube and received an automated email with ticket ref 147819 informing me that somebody will come back to me - which never happened.
Then again on 14/07/2022 Ncube advised my son that they are going to blacklist him if he does not make a payment. Because he is in the process of applying for permanent residency, he cannot afford to be black listed and AGAIN made a payment, this time into the account of Ncube Inc attorneys
My son called Ncube from Australia, but the receptionist could not assist him and he was told to forward a email, which wad done - yet again with a new ticket ref 166521 and no other response.
My son has thus paid the same outstanding amount to both Vodacom in 2020 as well as Ncube Inc in 2022. Vodacom did not forward proof of payment to Ncube and Ncube did not follow up payment with Vodacom. Vodacom sold their bad debt book to the attorneys and does not care what happens afterwards and Ncube, who bought the bad debt book does not bother to correspond with Vodacom either.
Either Vodacom or Ncube Inc needs to refund my son!