The Catch: Bluetooth bandwidth is limited. To support two-way communication, Windows automatically drops your audio from high-quality "Stereo" to low-bitrate "Hands-Free" mode. This results in: Muffled, distorted sound
This prevents Windows from ever switching to HFP mode. handsfree telephony windows 10
Sometimes, Windows 10 likes to "helpfully" re-enable this setting after a reboot or re-pairing. To kill it for good: The Catch: Bluetooth bandwidth is limited