How can the microphone work for speakerphone calls but not regular calls?

My Droid Turbo (Android v.4.4.4) has developed a problem. When I make calls nobody can hear me, but as soon as I hit the speakerphone button everything is fine. I'd take the phone back to Verizon for a replacement, but it's the same microphone in both situations, right? So how could it be a hardware problem? Is there maybe a volume setting somewhere I need to tweak?