Q. Which is the most correct phone number formatting—(xxx) xxx-xxxx, xxx-xxx-xxxx, or xxx.xxx.xxxx? Which is the most accessible?
A. You’ve punctuated your hypothetical telephone numbers—which are in the form most often used in the United States and Canada—in order of most familiar, most straightforward, and least conventional.
Putting the area code in parentheses is supposed to suggest that dialing it is optional. As area codes have increasingly become necessary even for local calls, this convention has nonetheless remained common.
As for accessibility, in our brief tests (using numerals rather than x’s), all three formats were read as phone numbers by both Microsoft Word’s Read Aloud feature and Microsoft’s Narrator—that is, as a series of ten individual digits with a pause after the third and sixth and not as three large numbers, two of them in the hundreds and one of them in the thousands. And each was automatically turned into a callable phone number link in various messaging and email apps on a smartphone.
But only the first two are mentioned in the recommendations published by the International Telecommunication Union, so we’d advise using one of those (the ones without periods). See also CMOS 9.57.