GOTOGATE does not respond - EVER

We booked my flights from South Africa to London and then London to the USA and return on all of them as well through Gotogate.

I flew with Virgin Airlines. We paid additional to have the option to change one of the flights if need be. I needed to change my flight from the Tuesday to the Saturday. Virgin said I have to make the change through the agency I booked the flights with. The number of Gotogate does not get answered. The online help centre does not respond. I could not reach a robot or a human being to assist me. My husband and I tried for 3 days to reach them without success. In future I would rather deal directly with the airline, then at least I would not waste my money on a company that does not even answer a phone call or return a email request. It is unbelievable that they are still in business.