I was on FNB/CellC and experienced this problem. I did a sim swap but the problem was still there. I then decided to port to Telkom but the problem is still the same.Have you recently ported?
I do receive calls but only SOME people have this problem. And they're on a variety of networks so I'm guessing theres some problem with my SIM.
Not sure what I'm missing here but will check the call forwarding settings.