Understand when user is speaking at the phone

Question

(Android) Some days ago I saw a way to register a broadcast receiver to understand when the screen goes black or the inverse.

Now i'd like to find a way to understand when the users is speaking at the phone but i don't find a broadcast for this case, so how can i understand if the user is at the phone? (Possibly without always running a service that would discharge the battery).

Tnk's

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