Why Is My Microphone Cutting out During Calls?

I'm having an intermittent problem where the microphone will cut out during a call. It is fine if I'm on speaker phone, but if I turn the speaker off and hold the phone up to my ear, sometimes the person on the other end cannot hear me. Sometimes, it seems as though it may be the angle the phone is being held at, but I never had this problem with my S4.

Is anyone else seeing this?