When you set an extension access number for a company, you need to only add an
extension number in the Contacts app for a contact at that company. The company's
phone number and extension are dialed automatically when you call that contact.
In the Phone app, swipe down
from the top of the screen.
Tap > Smart
In the Office Extension Access Number
field, type the phone number to access a company's extensions.
Tap any of the drop-down lists to change other smart dialing