The record
On September 19, 2014, a user writes that since moving their iPhone 5 to iOS 8, Siri could not hear their voice. They say other microphone features, including Voice Memos, FaceTime, Voice Control, and speakerphone, kept working, and Siri responded only when an Apple headset with a mic was connected. The poster tried a restore, a hard reset, toggling Siri, and turning Bluetooth off, and suspected an iOS 8 bug. The original poster did not return to confirm a fix, though other participants described their own workarounds. This is an individual account, not a measure of how many people were affected.
Read the original source at Apple Support Community (opens in a new tab) ↗
A documented user report, not an independently reproduced test or an estimate of how often the problem occurs.