There are a lot of ways to write telephone numbers, plenty of “standards” to chose from so why do so many people get them wrong?
For national numbers the only essential requirement is a space between the local part and the area code: that is 0117 9123456 (NOT 01179) for Bristol and 020 84641234 (NOT 0208) for London. Parentheses, hyphens and extra spaces can be put in for readability and local custom but it is not essential. A typical US number would be written (918) 555 1212.
On the web, it is often better to use international numbers. These should be written with a plus at the start, a space between the country code and the area code and another before the local part: i.e. +44 117 9123456. The ITU-T E.123 standard recommends that only spaces are used for extra separation. A typical French number (in Strasbourg) would be +33 388 12 34 56.