After pressing the Hello button, Google Assistant will ask the caller to leave a comment about the purpose of the call, and you can listen to the response in real time. If something important is heard, you can intervene and start a conversation. Otherwise, you can reject the call and continue working.
While Google Assistant has performed a similar function before, the Hello button gives the user full control over initiating the call prioritization process. There’s even an option to tell the caller to wait if you can’t answer temporarily. Additionally, the tool saves call transcripts for later review.
Source: Ferra

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