Unaccepted behaviour

Their accounts department send threatening and incorrect emails and do not respond to emails. When I received the account the service was provided a week ago. Their terms are 30 days but the email stated that the account is overdue and that 'several' attempts has been made to recover the money without any success and that the account will be handed over to lawyers and costs incurred will also be on-charged. This is not an isolated incident, my husband had exactly the same experience a few years ago I have sent two emails and submitted a complaint on their website - they have not responded