Lines not working!

Hi there. We have been MTN customers for years! My dad is actually a VIP member because he has been with you for over 20 years. Due to this we have also been with you for quite some time. But recently when I tried to phone my own mother I can't get through to her. My number is ********** 889 and hers is ********** 888. It either rings once and goes to voicemail or goes directly to voicemail. At times it will ring, but on her phone nothing happens. We tested it once where we sat next to each other. I showed her that I was calling her, and on my side it rang, but her phoned showed nothing. Did not even ring, showed a miss call - NOTHING. Please can you fix this as there is clearly something wrong and you are disconnecting me to communicate with my own family. She is my emergency contact and I cannot get hold of her at all due to this issue!