Here are a few things to try:

1. Check to see if the microphone is not being blocked by anything like dust, debris or a protective case
2. Restart your device in case the microphone trouble is caused by apps or software.
3. Check the phone's settings to make sure the microphone isn't muted.
4. Remove any accessories attached to the device such as a clip-on mic, external microphone or wireless headset.
5. Clean the microphone port and headset jack with compressed air.
6. Reinstall the phone's operating system software and drivers.