ITU-T Recommendation E.123, or the Notation for national and international telephone numbers Recommendation E.123 defines a standard way to write telephone numbers, e-mail addresses, and web addresses. It recommends the following formats (when dialling the area code is optional for local calling):

Telephone number, National notation (042) 123 4567

Telephone number, International notation +31 42 123 4567

E-mail address: name@provider.com

Internet address / URL: www.company.com

E.123 also recommends that a hypen (-), space ( ), or period (.) be used to visually separate groups of numbers. The parentheses are used to indicate digits that are sometimes not dialled. A slash (/) is used to indicate alternate numbers. This information is important if you want to make sure people know how to dial a phone number in a specific country.