It would be beneficial to know that voice assistant is listening to a message for several reasons. It can allow to give visual feedback that it is listening, or to send a WoL magic packet to wake up the machine running whisper, in order to reduce response time. This would be the case even when wake words are not used.
This is done in the pre-made M5 Echo sketch, it listens and changes the light color when it hears the wake word or you flip the switch.
Otherwise there is a switch in Ha that you can automate on as well.
Are you aware if there is a way of knowing if the assistant on a mobile phone is listening?