MSC CRUISES BAD SERVICE

I have made a booking online more than 2 months ago. I have received a Samsung voucher which I confirmed and verified with MSC as been valid to use with this booking. I have been speaking to Klaas, then Lindiwe, then Mpho and then also to Nosi. Not one of them is able to assist me to discount this voucher from my account, although they continue to say that it will be done! How can I escalate this problem? Is there anybody at MSC competent and able to assist? The service from your staff is really bad. Today the phone was put down twice as I tried to explain the issue yet again to someone at your call centre.