An international prefix, or exit code, is what a caller dials to leave their own country's network before the calling code of the country being called: 00 in most of the world, 011 in the North American plan, 0011 in Australia, 010 in Japan. It belongs to the way a number is dialled, not to the number, so the E.164 form replaces it with a plus sign. It is not the trunk prefix, the 0 (or the 1 in North America) dialled in front of a number inside its own country, which E.164 drops as well.
In libphonenumber 9.0.37's metadata, 185 of the 245 regions dial 00 to leave the country and 26 dial 011: the 25 regions of the North American plan and the Marshall Islands. Sixteen store a pattern of several prefixes with none preferred, and the rest have one of their own, such as Russia's 8, a pause for the tone, then 10. The phone number formatter lists every region's international and trunk prefix from the same metadata.
What Spaw answers
A prefix the lookup can read leaves no trace in the answer: e164 is the same with or without it. A number written with the international prefix of the country sent is read as international: 011 44 … with country: "US", 00 44 … with "GB". A leading 00 is also tried as a plus when no country is sent or the national reading fails, and kept when that gives a valid number.
When the input still does not parse as a number of the country sent, the answer is invalid and free, and did_you_mean carries the E.164 form of the one valid reading of two common mistakes: a missing plus, or a foreign international prefix left in (such as 011 sent with a country that does not dial it). 01112025550142 sent with country: "GB" answers too_long with did_you_mean: "+12025550142", and 447700900123 sent with "US" answers too_long with did_you_mean: "+447700900123". The suggestion is never applied. It is null when no reading or more than one is valid, when the reading lands in a block the regulator's table says nobody holds (unless the block sits in a range reserved for fiction), when the digits could be a national number with one digit too many or too few, for input that already starts with a plus, and for a short_number or emergency_number.
What it does not mean
did_you_mean is a reading of the digits, not a claim that the suggested number is in service or belongs to the person who typed it; looking the suggestion up is a separate lookup. A number whose prefix Spaw can already read answers valid with nothing to suggest, which is why the field is set only on invalid answers. Which prefix a country uses, and how its numbers are written for display, is on its page under phone facts by country.