The record
In a post dated September 19, 2013, a user says they had updated their iPhone 5 to iOS 7 that same day and found Siri would only respond when Raise to Speak was enabled and the phone was lifted to their ear. Pressing the Home button brought up the Siri screen, but it reportedly never registered speech and simply timed out. The poster lists toggling Siri, hard resetting, erasing settings, and rebooting, alone and in combination, without success. The thread received no replies and was closed with no fix recorded. It reflects one user's report only.
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A documented user report, not an independently reproduced test or an estimate of how often the problem occurs.