That's difficult, and dangerous at 70 mph in traffic.
Maybe you don't want to spend money on an add-on and Siri does not search add-ons for phone numbers to call anyway.
Here's what we did:
We set ACT! too allow the phone ext (and a pause!) within the phone field, so it will transfer to Outlook as part of the phone number.
Then we synced ACT! and Outlook...
Then we synced Outlook and the iPhone.
It's not perfect, but mission accomplished!
Contact me for more info and to set the Phone Field Option.