Imessage doesnt activate on my new sim.

I have been with MTN for a long time and recently upgraded my pay as you go to a contract with an iPhone 6S. However my iMessage on my iPhone is not activating on my number. I have tried every possible option to try fix it. I've restored my phone twice, and tried all the other solutions people have. I know its not the phone, because if i put my wife's sim card in the phone, it activates immediately and she is also with MTN. I put my sim in her phone, and it doesn't activate. I have done a sim swap and even that hasn't changed anything. Something is not working on MTN's side and no-one here in Port Elizabeth knows anything about iPhone's, and they only suggest things I've already tried. I was going to swap to Vodacom but decided to stay with MTN and right now I'm regretting that decision. And to note, my previous old iPhone with the same number on pay as you go had iMessage functioning without any glitches.