HD voice disabled itself somehow.

Hi,

My Iphone XR got a carrier update to enable HD voice calling, this worked great for a couple of days and then became disabled somehow. The sim in my iphone 7 (also an MTN sim)works with HD voice on both phones.

So I know this is a problem on the number (sim) and not the phone.

How do I re-anable HD voice on the number that it has been disabled on?