numbers are written down, especially in these days of ten digit
The ITU standard is "+country code area/city code local number" when you expect the number to be used by persons outside your country and "(ac) local number" for national only communication. Neither format uses hyphens.>>
ITU standards is different than what a country uses. +cc area code number with no punctuation is the ITU preferred format e.g. +1 311555 2368.
However, the standard way it's often shown in the US and Canada is (area code) NXX-XXXX e.g. (311) 555-2368. If you've seen any numbers by the phone company it's NXX NXX-XXXX i.e. area code space three numbers hyphen then four numbers e.g. 311 555-2368. Very often it's three numbers hyphen three numbers hyphen four numbers e.g.311-555-2368.
Other countries parse numbers differently. Many countries do area code hyphen followed by the directory number and depending on the number of digits in a local number. Some countries do not parse the numbers at all and just have the area code a hyphen followed by the local number. Personally I don't think people really concern themselves with how a number is written as long as it's easy for them to remember the number.
The French put their numbers in pairs e.g. for Paris 0123456789 they'd show 01-23 45 67 89 since they would quote a number as zero one, twenty-three, forty-five, sixty-seven, eighty-nine rather than two, three, four, five, six, seven, eight, nine.
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