A friend of mine asked me about this issue they found with their iPhone. They told me that they could make a phone call to any phone number from their iPhone even though it was locked and the screen was off. Just do a long press on the Home button and the Voice Dial pops up. And, low and behold, after a few tries we were making phone calls. I even noticed that it would show names of people that were in the contacts on the phone.
We found the setting to turn this on and off, and we seem to think this is enabled by default.
Go to – Settings > General > Passcode
There is a Voice Dial option on this screen, which turns this feature ON and OFF.
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Posted by Dayton 
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