International Telecommunications Union is the agency that provides for standards that telephone numbers should be using. Domestically, the government branch decides on the telephone number, however – the ITU will be the one to provide for the country dial code (such as +1 in the US). Any domestic number can not be more than 12 digits long. The NANP? is the only country code which has just a single digit as the country code. This might be important for some people to know.
When I type telephone numbers on my websites, I alway provide the full ITU standard telephone number with exception of placing a space between the country code, and the telephone number. This makes it easier for the person reading the number to have an idea of what country is being referenced just by looking up the country code.