These Bluetooth headsets have a physical mute switch that mutes the microphone at the hardware level, independent of the VoIP app. When you engage this switch, the VoIP app doesn't realize the microphone has been muted.
This is because the app mutes the microphone software, while the switch mutes the microphone hardware. So the app still shows the microphone as unmuted, even though it's actually muted.
This mismatch between the mute switch and the VoIP app's indicator is a common issue. We're working on a solution to sync the mute switch state with the app's microphone status.