Phone number E.164 format, often typed E164, is a plus sign, the country calling code, and the national number, in digits with nothing in between. Paste a list above and pick a default country: every number comes back in that one shape, with a note on what changed. Nothing you paste leaves your browser.
How to read the result
The default country only matters for numbers written the local way. A number that already starts with a plus sign or an international prefix keeps its own country.
Read the flagged lines first. A line is flagged when it has no digits, runs longer than E.164 allows, or is a North American number with the wrong number of digits. Look those up again rather than fix them by hand.
Most conversions turn on the trunk prefix. In most countries a leading zero is dialled only at home and drops out of the international form. In a few, Italy among them, the zero is part of the number and stays.
Store it, then use it
Store the E.164 form, and format numbers for people only where a person reads them. In a spreadsheet, set the column to plain text before you paste, or the plus sign may be read as a formula and long numbers turned into scientific notation.
For a mixed list, split it by country and run each part with its own default. Numbers that already start with a plus sign are safe in any pass.
The converter checks shape only. It cannot tell whether a line is in service, whether it is a mobile, or whether it still reaches the person on your list.
Still missing the numbers themselves? Our guide to finding direct dial phone numbers covers where they come from.