MTN has yet to repair my voicemail issue for 3 mth

I am an ex Nashua Mobile customer. From the beginning of November 2014 all Nashua Mobile accounts were handed back to MTN. From this time, I stopped receiving sms's when a voicemail gets left on my phone.<br> I have been calling MTN since the beginning of November and all that happens is I get to the call centre (after about a 30 minute wait), they look my fault reference number up, and say they will escalate the matter.<br> I have emailed the customer complaints email address - an auto-reply, but nothing more!!.<br> What more can I do to get this seemingly simple matter sorted out?.