E.164 format
Definition
The international standard way to write a phone number: a plus sign, country code and national number, no spaces.
E.164 format, explained
E.164 is the international numbering standard from the ITU that defines how phone numbers are written for global routing. An E.164 number starts with a plus sign, followed by the country code and the national number, with no spaces, dashes or leading trunk zero, and at most 15 digits. A New York number looks like +12125550123, and a UK mobile like +447700900123, dropping the 0 used for domestic dialing. Many apps store numbers this way, so if a form rejects your number, try entering it without the leading 0 and with the correct country selected.