If you are on MTN and can't call Cell C, then MTN should resolve the issue or make other parties aware of the problem.
However noting that Cell C to MTN calls work, Then Cell C should not need to bare responsibility.
Basically who ever you pay, for the call should fix the problem.
I Had the same issue with Telkom not being able to call VOX 087 numbers.
But Vox was able to call Telkom numbers with no problems.
Therefore Telkom should bare responsibility, but they did not and Telkom advised to contact VOX.
When explaining to Telkom that I pay them (Telkom) for the call and therefore it is their responsibility to rectify, only then did the penny drop.
Sorry for Hijacking the thread, just posting my past experience, In a nut shell this is MTN's problems and not Cell C.
Bringing me back to Cell C's comment which I stand by 100%
"The customer should raise the issue with his or her current network provider. It is the responsibility of the network provider to investigate the matter and if necessary take it further with the customer’s previous/former network provider."