What is the function of the blue circle during a call on an iPhone?

On my iPhone SE, using iOS 16.6.1, I am using WhatsApp. I received an audio call. I could not answer it. I noted a plain blue circle at the upper right corner of the screen, about 12mm in diameter. It had no image inside of it, such as a microphone. Inside was a white checkmark. I attempted to answer the call. I tapped that circle. Nothing happened. Eventually I was able to answer it, by tapping something else. What is the meaning and function of the blue circle?

[Re-titled by Moderator]