Dialup is not so widely used these days, however with all the recent hacking going on it may be safer to make a secure direct connection.
If you’re configuring dial-up manually or using the unattend.xml then you can automate this by using these settings.
In the customsettings.ini enter:
AreaCode=0161 CountryCode=044 LongDistanceAccess=9 Dialing=TONE
Dialing can also be set to PULSE. For a list of XP country codes look here.