Welcome to Android Central! Which phone? Have you tried using the speakerphone function, or wired or Bluetooth headsets? If the other person can hear you that way, then the phone's primary mic is either blocked (e.g., by lint or other gunk) or faulty. Inspect the microphone opening closely, and clean it out gently if it's blocked. If it appears clear and the other person still can't hear you, then contact the phone manufacturer (if still under warranty), or bring the phone to your local repair shop.
09-28-2016 02:35 AM