They refuse to fix my problem

I ported from MTN to Cell C at the beginning of my current contract. Since doing this I have been unable to receive phone calls from Vodacom numbers. If a person calls me from a Vodacom number, they are greeted with a automated message saying something to along the lines of "the number you dialed is not available on the MTN network". This problem went unnoticed for a while as it seems pretty arbitrary and it took a family member mentioning offhand that this had been happening and that it had happened for a long time.

After a repeated phone calls to the "help" line, I have been told either A) not their problem, contact MTN (who then tell me I am not their customer so contact CellC instead) or B) that the problem is fixed or will be fixed the following day (and it never has been). I have been trying to fix this problem for over a year now.

I cant even tell how much it has cost me as I relying on being reachable via my phone for work. I am terrified to port to another network and have this problem possibly compound further, because Cell C refuses to properly fault find this problem or contact MTN as the mix up may be there, but ultimately I am Cell C's customer and the onus of fixing this is on them. I fear that I may have to abandon this number that I have used for 20+ years just to be reachable by any service provider. I will never use Cell C again