during phone call the other side hears an echo of herself when the L5 side is talking simultaniously (when "interrupting")
you could argue the L5 highlight bad habits and enforces a conversation exercise to listen to your partner until she finishes, but noises can still happen by accident while the other party is talking :)
and exactly in this case, the other party suddenly starts to hear an echo of herself. It ends again when the other party is talking alone again.
So that indicates that our echo cancellation works but shows hour tricky that can become. When the signal on the mic is not obviously similar to what is received, echo cancellation currently fails.