Dialing rules are located in the registry at:

HKEY_LOCAL_MACHINE\SOFTWARE\Microsoft\Windows\CurrentVersion\Telephony\Country List.

For each configured country, the international country prefix sub-key contains information about the rules. For the United States, prefix 1, the Value Names and data values are:

    Value Name     Data Type     Data Value
    Name     REG_SZ     United States    
    NameResourceId     REG_DWORD         00006979
    CountryCode     REG_DWORD     00000001
    SameAreaRule     REG_SZ     G
    LongDistanceRule     REG_SZ     N1FG
    InternationalRule         REG_SZ     S011EFG

You may change the InternationalRule and LongDistanceRule data values, using the following codes:

0-9 Numbers to be dialed.
ABCD       Characters to be dialed (tone dialing only, used for special control on some phone systems).
E Dial the country code.
F Dial the area code (city code).
G Dial the local phone number.
H Dial the card number.
* # Characters to be dialed (tone dialing only).
T Subsequent numbers are to be tone dialed.
P Subsequent numbers are to be pulse dialed.
, Pause for a fixed time.
! Flash (half-second on-hook, half-second off-hook).
W Wait for a second dial tone.
@ Wait for quiet answer (ring-back followed by five seconds of silence).
$ Wait for calling card prompt tone.
? Suspend dialing until user provides input.
L Mandatory long-distance carrier.
M Mandatory international carrier.
N Optional long-distance carrier.
S Optional international carrier.

NOTE: See Windows XP Help and Support. Type dialing rules into the Search box and press Enter.