Setting Speed Dial
Open the Contacts app, then tap the Menu icon
(at the upper-right corner of the screen).Tap Speed dial.
Tap the speed dial number to assign, then tap the Contact entry for it.
Repeat selecting a speed dial number and the desired Contact entry as necessary.
Setting Speed Dial for contact entries with multiple saved numbers
When you use the Menu icon (at the upper-right
corner of the screen) to set a speed dial, the default number is automatically
selected. If you choose the same entry for a speed dial again, the
next saved number is automatically set for the speed dial.
The Speed dial icon 
in the contact entry allows you to set speed dials for selected
phone numbers.
From the contact entry (either when creating it or while editing it later), tap the Speed dial icon
to the right of the phone
number you want to set with a speed dial.Tap the speed dial number to assign to it.
Tap Save (at the bottom of the screen).