Account Paid 2 years ago listed as In-Arrears!!!!

An account that was paid up 2 years ago is listed as in arrears and therefore affecting my credit score!!!!! Now I'm struggling to buy house because of this!!! I've called the customer line, nobody picks up, I've sent dispute e-mails with paid up letter, no response!! Please resolve urgently!!!!!! You are affecting people's livelihoods!!!!