Telkom service unacceptable

My son app**** for the lockdown special they had for adsl in June 2020, being R399.00 for the first six months, then R699.00 thereafter. Since inception of this contract we had difficulty in rectifying this account, as they keep on billing for R699.00. Last month it was by some miracle rectified with the necessary credits, only to be billed wrong again for this month. We are at our wits end. Their service sucks!!! Telkom is defaulting in the contract, not my son.