What does it take from a customer to activate HD Voice VoLTE?

I ported over from Vodacom to MTN the porting was completed on the 25th of June and everything works as expected except getting VoLTE (HD voice) to work??

I have emailed customer care twice (No response) phoned 135 on a couple of occasions to ask for it to be activated either they tell me to switch my phone off for 5 mins or to phone the porting department or go to an MTN store (who was of little help)

When I place or receive a phone call my iPhone it will always jump back to 3G instead of using Voice over LTE. This function always worked on Vodacom?

https://www.mtn.co.za/Pages/MTN-High-Definition-Voice.aspx#:~:text=MTN%20High%20Definition%20Voice%20uses,on%20SA's%20fastest%20mobile%20network*.