Go to the Contact in the Address Book.
Scroll to the phone number onto which you wish to place the extension.
Scroll to the end of the number. (ie after the 555-123-1234 place your cursor).
Press the Menu button.
Select Add Wait if you want to be prompted to manually press the trackball to SEND the extension numbers which you will enter next.
Select Add Pause if you want to have the device automatically send the digits after a prescribed period. NOTE, you may need to add MULTIPLE PAUSES in order have the pause be long enough for your phone system.
After the Wait or Pauses are entered, type in the numbers of the extension.
SAVE your changes.
REPEAT as needed.