MTN is the WORST

I've been experiencing ongoing problems with people not being able to reach me. My phone switches straight to voicemail. I'm able to make calls, send and receive SMS's, use WhatsApp, but I AM UNABLE TO RECEIVE PHONE CALLS!!!!!! Across ALL networks - Telkom, MTN, Vodacom, Cell C. I can't go on like this - what use is having a cell phone and people can't reach you and you eventually end up in screaming matches because people can't get hold of me?