E.164
The international standard phone number format: a plus sign, country calling code, and national number, with no spaces or punctuation.
E.164 defines a globally unambiguous way to write a number, up to fifteen digits. The trunk prefix that people dial domestically, the leading zero in most of Europe, is removed. A UK number written 07400 123456 becomes +447400123456.
It is what carriers, messaging platforms, and telephony APIs expect. Storing anything else means every downstream system has to guess which country the number belongs to.
Converting to E.164 is not string manipulation. The trunk prefix, the national number length, and whether a leading zero survives all vary by country, so it requires numbering-plan data rather than a regular expression.