Call Forwarding Spinning Wheel

Hi. I have an iPhone 12 and I recently changed providers to Belong. Previously, when I was with Vodafone, I could select Settings>Phone>Call Forwarding and divert calls to my work mobile (iPhone 8) during the week. Since joining Belong I can no longer do this. When I go into Call Forwarding all I get is the spinning wheel. Interestingly, if I switch my work sim (Optus) into my iPhone 12 I can use Call Forwarding. No spinning wheel.

I have raised this issue with Belong and they say it is not a problem on their side and that I need to consult my phone manufacturer.

My iPhone software is up to date.

Does anyone have any ideas or solutions on how I might be able to resolve this issue?

Thanks