Q. Telephone numbers . . . what format does CMOS prefer? (800) 555-1212 or 800.555.1212 or 800/555-1212. What if the country code is required?
A. We prefer hyphens or hyphens and parentheses, as follows (though you may choose to leave out “1-”—the country code for the United States and Canada):
1-212-555-1212 or (1-212) 555-1212
But unless you are constrained by, for example, an Internet form, it’s okay to use periods or slashes or even small bullets if you prefer. Any hyphens, parentheses, spaces, periods, etc. are aids to understanding the separate elements in a phone number. Where these aids are is more important than what they are; one does not, for example, dial (or more likely press) a parenthesis or a hyphen.
For international numbers, include the country code preceded by a plus (which indicates that additional numbers, depending on the origin of the call, are required). For example, a French number looks like this—
+33 1 00 00 00 00
—where “33” is the country code for France, “1” designates the Île-de-France region (which includes Paris), and the rest is the usual eight paired digits separated by spaces for French telephone numbers. From the United States, the plus sign signals the 011 needed to initiate an international call; from many other countries, it’s 00.