late payment

There are invoices that are sent a long time ago through their ICE system, the invoices were never changed at all for being ready to pay since September till today they never pay the invoices. we also phone them today to tell them that their system shows that some of our invoices are paid but we never received payment of those invoices. we sent an email on Wednesday concerning matter of payments but nothing happened. we also sent another email yesterday and still they don't reply to tell us the report. when we phone them they keep on transfer our call from one person to another person and keep us holding till they put our call down. so we have no choice but to report this matter because they don't reply our emails and we can't get help through the phone