Tap the back button.
Press the Sleep/Wake button or the Home button 
.
Use the keypad to dial a non-emergency number.
Make an emergency call when iPhone is locked: On the Enter Passcode screen, tap Emergency Call.
Important: iPhone can be used to make an emergency call in many locations, provided that cellular service is available, but it should not be relied on for emergencies. Some cellular networks may not accept an emergency call from iPhone if iPhone is not activated, if iPhone is not compatible with or configured to operate on a particular cellular network, or (when applicable) if iPhone does not have a SIM card or if the SIM card is PIN-locked.
In the U.S., location information (if available) is provided to emergency service providers when you dial 911.
With CDMA, when an emergency call ends, iPhone enters emergency call mode for a few minutes to allow a call back from emergency services. During this time, data transmission and text messages are blocked.
Exit emergency call mode (CDMA): Do one of the following:
Tap the back button.
Press the Sleep/Wake button or the Home button .
Use the keypad to dial a non-emergency number.