I am trying to call 100 to get voicemail messages on MTN and I get a recorded message saying "You have reached an invalid number". Dialing 111 gives me the same message.
Finally got through to someone at customer support who knew what they were doing. Something was not loaded when I ported from contract to prepaid and my mailbox needed to be reset.
I also struggled with the MTN muppets. over the disabled voice-mail after porting to payg.
You can remove and then add voice-mail back to your number by dialing 141 and follow the prompts.