Ethiopian airlines unreachable

I need to have my tickets changed (due to Covid). I have tried to contact their call center in Johannesburg 15 times (different days). Each time I was told to hold the line because all there agents are busy. I kept on holding for more than five minutes and then just got cut off. I have also tried contacting their global call center- without success. How are we supposed to do business with them? No other contacting methods are available.