Make sure headset or headphones are plugged in correctly with simple Headset Indicator.
Headset Indicator makes status bar notification when a headset or headphones are connected.
If supported by device, notification shows if microphone is available.
This app is especially useful for some custom ROMs, devices doesn't have default notifier like some Xperia, Nexus devices.
Be sure to ENABLE notification, open app then click "Enable Notification"
Hide App icon from launcher (optional)