BA can't be trusted

I am so disappointed and frustrated with BA! My sister's connecting flight from Johannesburg to Durban was meant to land in South Africa, Durban in the morning of the 24th December. Now they have changed it to 6pm! We were not notified about why the flight was changed and when we check the website there are still seats available on the earlier flight! We have tried numerous times to contact the airline to no avail!!!