Vodacom would have lost out the most on revenue as they have more net inbound calls. MTN would have a slight net loss as they would pay more to Vodacom but get less from Telkom. Cell C may lose out overall.
Not sure of your grounds for making above statements and or source of info for coming to your conclusions
However unless you have inside information indicating different calling patterns between the different operators I suspect you could be drawing the wrong conclusion. The absolute level of net inbound calls does not make a difference as long as the average calling patterns of each operator is the same and the MTR is symmetrical. Vodacom's loss due to the reduction in MTR is likely to the highest followed by MTN and then CellC but primarily due to Telkom's (and possibly the VOIP operator's) gains and not due to pmts between the 3 mobile operators.