Can’t make Outgoing calls on my Mtn sim

Hi I’m currently on an Mtn contract for many years... 1 week ago , out of no where , I tried making a call, and my call just cut and said “call ended” I’ve tried calling many different numbers , and same results

I tried calling the Mtn service providers, everything is clear on my account side , they suggested to deactivate my call barring , switch off the phone(iPhone 6s) and wait, I’ve attempted that many times and still... same results. I then went further , and went to the Mtn Overport city branch, hoping they could assist me, but unfortunately, as much as they tried , they could not help me, while I was there , another lady had the exact same problem

This is a major concern for me , as I need to make calls, I can receive calls, and use my data freely, I just cannot make outgoing calls

I’ve even tried putting another network sim in my phone , and that works 100%, so obviously it’s on Mtn’s side

Can anyone please help and assist me?!