Cannot make phone calls with the "Phone" app

Hello,

I have an iPhone XS Max, 64GB running on IOS 12.1

When i call someone the only thing they're hearing is a loud crackling noise. They cannot hear me at all, i can hear them perfectly though. And the most irritating thing is the fact that sometimes it works just fine. So everytime i call someone i have to ask "can you hear me?'' first... I've been to an Apple store for this issue about a month ago, they ''Diagnosed'' it and there were no issues with the hardware. The Apple guy told me to factory reset my iPhone and i did. After the reset i must say it worked just fine for about 3 weeks. But now since a week ago the exact same issue occured. I don't want to have to reset my iPhone back to factory settings everytime this happens. Oh and by the way, everything works perfect on the other apps like Facetime and Whatsapp calling. So it's just the phone app. It may be the carrier but i've installed a new SIM the same day i went to the Apple store so i doubt it.

PLEASE HELP

Greetings,

Burak