Phone call issue

I have a Samsung Galaxy S21 FE. Contract finished with Three so have now switched to Smarty.

Ever since I bought this phone I've had issues where people on the other end of my calls struggle to hear me. Though only for certain customer service lines. If it's friends or family then it's always crystal clear for them. I was hoping changing to Smarty would fix this but it hasn't.

Anyone has any idea what could be going on?