Cell C's Voice Call Quality is Horrific

In 2013 I arranged with Cell C to have my wife's MTN cell number ***) to be ported, or transferred as I wanted to change network providers. For the first year we had no problem with the quality of the Voice Calls, and it only became apparent at around August 2014 that the quality of the Voice Calls was getting worse, my family inform me that I keep breaking up and half the time the calls just drop. I was told to upgrade my contract with a new Cell C sim in June 2015 and the problem would get resolved, well guess what? Nothing has changed, Cell C then said I must contact their technical department who seem to keep passing the buck due to the number portability. Cell C please wake up and take responsibility, once the number is ported, it becomes your responsibility to ensure the quality of the Voice Calls are as good as a traditional (084) number. Please get this problem resolved as my the quality of the Voice Calls is terrible and if it doesn't get solved I will pursue options CPA. The number is ***.