keep having phones swapped out by my carrier, on 3rd iphone in one month, no one can hear ME in phone conversations.. problems with 8plus and 7plus, its an APPLE speaker problem that seems yo be unresolved

People can’t hear ME on phone conversations, they say my voice is muffled.. I don’t have a cover and I’m careful on how I hold the phone. I’ve had 2 8plus and now I’m with a 7plus all having same issues... I can’t hear the problem b/c it’s on the rec’ing end of call... very frustrated with everyone complaining. Speaker phone seems to be the only resolve. I feel it’s an APPLE problem they don’t know how to fix. My carrier has been great with trying to address the problem and they say a lot of phones are having that problem. Maybe I need to try the iphoneX