Hu ZyraBot 
I think you need to wait some days to correct the issue.
Phone porting and success voicemail porting sometimes takes days.
Have a nice day It's been a pleasure to assist you today 
If u love my answer don't forget to mark as the best answer
Hello ZyraBot,
Sorry to hear you're having issues setting up your voicemail on your pre-paid account. Since you've migrated from a post-paid to pre-paid account, you might need to contact customer service to have them reset your voicemail.
What pre-paid plan do you have? You should note that some of the pre-paid plans only have voicemail on a pay-per-use basis. You should also note that airtime (plus long-distance or roaming charges, if applicable) would be incurred for listening to your messages (see here).
Hope this helps 😀
Cheers
