It's stored in HKLM\SOFTWARE\Microsoft\AddrBook\Default_Area which is of type REG_SZ and set to 425.

Changing it does not require a reset.

From my own customize.sa.cab in extended rom:

[HKEY_LOCAL_MACHINE\SOFTWARE\APPS\Microsoft Application Installer]
"nDynamicDelete"=dword:0

[HKEY_LOCAL_MACHINE\SOFTWARE\Microsoft\AddrBook]
"Default_Area"="520"

[HKEY_LOCAL_MACHINE\Control_Panel\Phone]
"Flags2"=dword:10


Ehud