MTN Failure/refusal to rectify Voicemail Lite problem.

I have an MTN Contract, approximately in September 2020, I changed my voicemail service to voicemail lite, this will not allow callers to leave a voicemail but will inform them that I am not available and an sms would be sent to me informing me of the call I missed. However, this does not happen, instead, when a caller calls me and i do not answer or my phone is off, the caller hears an automated message that they have dialled an invalid number. This is a huge concern for me as I use the phone for business and am unable to call a caller back if their number was withheld, furthermore, callers will believe that i cannot be reached on this number any longer. I have reported this to MTN several times since i became aware of the problem, all i get are reference numbers and that's it, no resolution, no feed back, nothing. On Friday the 8th of january 2021 i wrote an email to MTN Customer services, they acknowledged receipt and said that they would respond within 48 hours. I sent a follow up email on Monday, again, I heard nothing from MTN and the problem still exists.