Call Failed problem in Iphone x

I bought an iPhone X factory unlocked from USA

I am in Pakistan and using in Pakistan from Januray. I unable to talk to anyone, call failing after few seconds while talking. Not all calls, several calls failing.

Is it the expensive mobile in the market and facing this type of issue is ridiculous

Did everything, no network problems! Tried changing sim, did reset network settings! Anyone out there to solve this issue? Or do I need to change the phone