Update for iOS 12With iOS 12 and iPhones that have Touch ID, you can still bypass the iPhone lock screen and trick Siri into getting into a person's phone.
The bypass is the same as it was in earlier versions of the operating system:Press the home button using a finger not associated with your fingerprint authentication, prompting Siri to wake up.Say to Siri: Cellular data.Siri then opens the cellular data settings where you can turn off cellular data.[ Further reading: How to use a strong passcode to better secure your iPhone ]As was the case before, anyone can do this.
It doesn't have to be the person who "trained" Siri.