A contact can be saved on a number key and dialed by tapping and holding the key it's saved on. It's easy, but remember you need to save a contact on a speed dialing key before you can use this function.
Tap the phone icon.
Tap Dialling Pad.
Tap and hold the required speed dialing key to make a call.
Tap End Call.
Slide your finger upwards starting from the bottom edge of the display to return to the home screen.