Program Personal Speed-Dial Buttons
Once an administrator has defined one or more speed-dial instances for a Cisco Unified IP phone, you can reprogram numbers that are not locked or can program numbers into an instance that has an empty dial string by following these steps:
Step 1 Select an available phone line by lifting the handset, by pressing the NewCall soft key, or by pressing a button. Listen for the dial tone.
Step 2 Press the pound key (#).
Step 3 Press the Menu button, press 4 on the dialpad for Speed Dial and use the Navigaion button to select the speed-dial button that you want to program. A short beep confirms that you are starting to program this button.
Step 4 Enter the speed-dial number. The digits will appear on the phone display. When you are entering speed-dial numbers on a Cisco Unified IP Phone 7912G, use the backspace (<<) soft key to erase digits that were entered incorrectly.
To remove a speed-dial number without replacing it with a new one, press the pound key (#).
Step 5 Press the same speed-dial button a second time to indicate that you have finished entering the speed-dial digits and to store the new speed-dial number.
Step 6 Hang up the handset, or press a new speed-dial button and repeat the process.
http://www.cisco.com/en/US/products/hw/phones/ps379/products_user_guide_chapter09186a00801aea8f.html#wp1047631
