Harassment from this company is beyond

The school handed over the account to this company in August 2021 and full payment was made to the school in December 2021 the whole amount was paid. So this company is harassing me with calls they said I must pay interest fees and Legal fees from when the account was handed over to them up to date..after making payment to the school I sent this company a proof of payment the school confirmed that the account is paid up and there is zero outstanding fees.. But now the debt collector is harassing and threatening me for the account that was paid up last year..I don't understand is this aligned with Debt collection ombudsman,their consultants are very rude they unable to explain the charges.. Are they allowed to charge me for an account that is fully paid up?