You can't. Masking of phone numbers in Outlook is hard coded.

Signature
Russ Valentine
[MVP-Outlook]
Russ, thanks for your response.
Any idea why it does allow me to hyphenate the area code on Outlook 2003 on
my work computer, but doesn't allow my secretary to do it on hers (same
version), and doesn't allow me to do it on my home version?
> You can't. Masking of phone numbers in Outlook is hard coded.
> > How can you stop Outlook from automatically format an area code with
> > parentheses? I want to use a hyphen instead.
> >
> > Outlook 2003
Russ Valentine [MVP-Outlook] - 09 Mar 2007 20:57 GMT
It's certainly not be design. On some versions, if your default dialing
location is not set correctly, the automatic masking is not applied. It's
not a recommended workaround, however, because Outlook will eventually start
nagging you incessantly if you don't set a default dialing location.

Signature
Russ Valentine
[MVP-Outlook]
> Russ, thanks for your response.
>
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>> >
>> > Outlook 2003