International Dialing
Pick where you are calling from and where you are calling to. The exit code is the half that varies — it is 011 in the US, 0011 in Australia, 010 in Japan — so both ends matter.
011+44+local number
From the United States to the United Kingdom: dial 011, then 44, then the local number without its leading 0. Change either menu above for any other pair.
Exit prefixes, trunk prefixes and example numbers come from Google's libphonenumber metadata (Apache-2.0), joined to the ITU-T E.164 calling codes. Where a country selects its international prefix by carrier, this says so instead of picking one.