Sorry, you can't. Masking of phone numbers is hard coded in Outlook.

Signature
Russ Valentine
[MVP-Outlook]
>I enter phone numbers as xxx.xxx.xxxx They are then being automatically
> converted to (xxx) xxx-xxxx. How do I turn off that feature? I do not
> want
> the phone numbers displayed that way, I would like them to display the way
> I
> entered them. Thanks.