Call Forwarding Issue | calls are not working

I can’t received and make calls for months now when I use my simcard on my Iphone 8Plus. But when I insert my simcard to other phones, it works. I already called the carrier service provider and the problem is on my phone. The call forwarding is automatic turned on. I can’t also turn it off in the Settings and just keeps loading (see photo). Codes on how to turn off call forwarding is not also working. Is this something to deal with the IOS or old iPhone models? Please help me. :(