MTN is driving me nuts

Ever since switching to a Samsung Galaxy in January, I have had problems with my Voicemail.

I posted about this a few weeks back and somebody on this forum got it up and running for the first time in 8 months............and now it's gone again - worked for a while and once again - no voicemail.

Please could someone help here.

Dialling 808 is useless, as it rings for 15 minutes, and then the phone cuts out.
MTN shop cannot help, as they cannot get through either!

HELP!!