Some users have reported an issue in which Voice Control spontaneously engages on various iPhone models (including the iPhone 3GS and iPhone 4). The problem appears especially prone to occur while the user is actively moving, such as when running.
The problem may occur only with certain headphones or headsets. Users have reported the issue with Apple in-ear headphones, V-Moda Vibe Duos, Etymotics HF2 and others.
Reset all settings. Navigate to Settings > General > Reset > Reset All Settings. This workaround may only temporarily resolve the problem.
Block the mic. Blocking the microphone on Apple’s or third-party headphones, by adhering tape or via other means, (albeit kludgey) appears to resolve this issue in some cases.