SLA not being adhered

DSTV customer service is utterly poor, they don't adhere to their own SLA. I made a payment using the incorrect reference (my mistake). I tried several times to email them the correct proof of payment but my email was rejected as their inbox was full, my email finally went through as I received an acknowledgement from them stating that they will get back to me within 24 working hours which is 3 working days. I sent an email on Monday and today it's Thursday and they still haven't gotten back to me. What does one need to do to get proper customer customer service?