Holding down the iPhone 4S home button works to call on Siri even if the phone is locked. No need to unlock and possibly type in your device password.

Update: If you use a passcode on your device and want to disable the ability to call on Siri from the Lock Screen, go to Settings > General > Passcode Lock, the option is there.

Disable Siri on Lock Screen

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Little features that could make a big difference in iOS and iPadOS

I have a growing list of little features that I hope Apple will add to iOS someday. Since WWDC 2021 was not that day, I’ll share them here.