The record
In a thread opened September 16, 2015, a user says Siri on their iPhone 6 behaved normally through the car's Bluetooth but, away from it, kept saying it did not understand and offering suggestions, while dictation did not respond. They report cleaning the microphones and connectors, resetting network settings, toggling Siri and Hey Siri, restarting, calling Apple support, and restoring to iOS 9. A day later the poster said Apple had replaced the phone, and afterward they still noticed a missing activation tone, which another participant attributed to a design change. This describes one user's experience and is not evidence of a wider fault.
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A documented user report, not an independently reproduced test or an estimate of how often the problem occurs.