If you setup your stereo headset via "Bluetooth Settings -> Express Mode" only "A2DP profile" (used for stereo music listening) is configured. If your stereo headset has a built-in mic, it will also support Headset Profile (HSP) or Handsfree Profile (HFP). Those profiles offer mono audio but microphone support for using VoIP applications. In this case, you have to set up a new profile for the device via "Bluetooth Settings => Custom Mode" and select HSP or HFP. After it pairs, go to Bluetooth Settings window, right-click on the new profile and select 'Connect'.
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