Some units may have Bluetooth microphones built into the front of the receiver or external microphones.
Many Bluetooth microphone issues have to do with the user’s proximity to the microphone or with road noise (e.g. windows, sunroof, etc.). Try experimenting with different microphone placements and closing any open windows. Additionally, some units have microphone volume control which can adjusted manually to improve the audio.
For external Bluetooth microphone issues, please confirm the mic is fully inserted into the 3.5mm mic input.