Pathetic MTN strikes again

MTN keep processing a debit on my husband's account. He paid the amount quoted by MTN themselves on 4 February 2020. He called to confirm payment was made (a painful system they have which is call for the figure, go pay instore then call again to confirm) as per their instructions. Since then he has had his account debited at month end every month (which he reverses). Querying has yielded two responses of 'apologies the account has been paid and is now pay as you go this won't happen again' and a final response of 'the amount MTN QUOTED IS WRONG.... How, does a company get to give a settlement figure, confirm it more than once THEN decide oops sorry that was wrong and continue to debit your account as they please? What is the point of the settlement if they don't give you the correct value to start with (which is for interest sake virtually impossible to figure out yourself)