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

Signature
Russ Valentine
[MVP-Outlook]
> The telphone numbers for all my Outlook contacts are displayed in a wrong
> way
[quoted text clipped - 9 lines]
> uploaded both with country and area code - meaning I can't call any of my
> contacts. How do I avoid the area code?
If your phone numbers are entered in Outlook in full international format and
synched, then your cell (mobile) phone should dial them correctly. I have
never encountered a phone that won't. Correct format in your case is: +45
nnnnnnnn.
The format you are getting suggests that you haven't properly set up your
Windows dialing location, in your case Denmark (country code 45).
> The telphone numbers for all my Outlook contacts are displayed in a wrong way
> because of the City/area code list in the telephone number.
[quoted text clipped - 5 lines]
> uploaded both with country and area code - meaning I can't call any of my
> contacts. How do I avoid the area code?
Russ Valentine [MVP-Outlook] - 12 Apr 2007 23:11 GMT
Correct, in theory. Outlook must present a consistent and standard phone
number format since it is the database to which other applications can
synch. And that's exactly what it does.
Your assumption that all phone synch software recognizes and abides by that
standard may not be correct, however. Judging by the number of posts in this
group asking Outlook to change its format so their phones will work, I
suspect that assumption is not correct.

Signature
Russ Valentine
[MVP-Outlook]
> If your phone numbers are entered in Outlook in full international format
> and
[quoted text clipped - 18 lines]
>> uploaded both with country and area code - meaning I can't call any of my
>> contacts. How do I avoid the area code?