How can I use bluetooth earphones with the iPhone microphone (and NOT the mic that is built in to the earphones)?

To use Bluetooth earphones while utilizing the iphone's built-in microphone, you can disconnect the Bluetooth connection. Go to Settings > Bluetooth and turn off Bluetooth to disconnect the earphones. Alternatively, you can also select the iphone's microphone as the audio input source in your app settings, if the app allows for such a selection. Keep in mind that this may vary by app, and not all apps might support this functionality.