Customer service not available to South Africa

I booked a return flight from Durban to Johannesburg, with a flexi ticket that allows me to change me flight at any time, however it has been impossible to get in contact with their customer service center to change my flight, with their number no working for South African flights and an email address that is never answered. At this point is seems they made people pay for a service they have no way of providing as they’ve made it impossible to contact them.