Everything goes to voicemail

I have now spoken to people at 2 MTN shops and 3 call centre agents and still I am unable to make or receive calls it either doesn't ring or goes straight to voicemail, absolutely frustrating when I use this number for work purposes and I receive an sms saying I have a new voice message when I'm sitting with the phone in my hand with 4 signal bars on 4G+, firstly it's not the phone I've tried the sim in another phone and it does the same thing, I keep getting told to dail #330*0000# it doesn't fix anything, had this problem since July 2020 and it's still not fixed.