Voicemail not retrievable. Customer service not helping

I have called the customer care so many times over the last year. I have a contract with MTN, but have not been able access my voicemail for more than a year. The problem has never been resolved. How can calls be closed and no one asks if I can now access my voicemail? Every time I call, the technicians reset it, and nothing changes. I use my phone for work. When I try to go to the next voicemail it tells me invalid option on all the options. Call centre agents just put me back in the queue when I ask the matter to be escalated. Why are you not taking my matter seriously? This is such bad service. No one cares.