Personally, I was very sceptical of phones listening. But this paper on [1] "Ultrasonic Device Tracking", has convinced me that the technology is definitely possible.

Apparently it's known as ultrasonic malware and is a type of side-channel attack to leak information to/from devices. So high-frequency audio is transmitted to devices that are listening and can even respond.

The scary thing is that researchers have already found this type of software embedded into [2] many applications.

Now this isn't the same as listening to "human conversations", but it's not hard to believe that some apps are waiting for any audio and just sending it to servers somewhere...

What apps do you have installed on your phone? Any games? Apparently [3] some games listen to you, even in the background...

[1] https://intellisec.de/pubs/2016-batmobile.pdf

[2] https://www.bleepingcomputer.com/news/security/234-android-a...

[3] https://www.nytimes.com/2017/12/28/business/media/alphonso-a...