Dropping calls

Have iPhone Xs and constantly dropping voice calls, doesn’t matter if I placed the call or received a phone call. After few minutes I can hear two beeps and call is dropped. If I recalled straight away have failed call. It’s extremely annoying, went to Apple store and they advised to switched off wi-fi calling, also I changed the SIM card, checked settings with Optus, soft reset the phone which only helps for a day and the issue returns. I even did the hard reset, wiped out all data and still the same problem. Can’t be the carrier as my partner has exactly the same provider and never have any dropouts on his Samsung s9. Seems like it’s an Apple problem, the phone is to expensive to have this problems.