You can make a direct international telephone call from your home or from designated public telephone without signing a contract with a long-distance company. Access codes, rates, services, and discounts differ depending on the international telephone phone company you use.

★If you are calling from a land line and are registered with "My Line":

Dial 010+ country code + phone number with area code minus the first zero

 *For example calling the US from Japan with MY Line you would dial 010-1-646-555-1234

★If you are calling from a land line or public pay phone and are not registered with "My Line":

Dial Carrier Code + 010+ country code + phone number with area code minus the first zero

NTT Communications - 0033
KDDI - 001
Softbank Telecom - 0041

 * For example calling the US from Japan with Softbank Telecom you would dial 0041-010-1-646-555-1234

★Calling international from a mobile phone
Depending upon your mobile contract pre-registration may be required

Carrier Code + 010+ country code + phone number with area code minus the first zero

DOCOMO - 009310
AU - 005345
Softbank - no code required
Willcom - no code required

★Country Codes
US / Canada 1
UK 44
Brazil 55
Australia 61
Japan 81

Full list available here: http://countrycode.org/

Source: http://www.iwami.or.jp/fudoplan/first-time.htm