How to change the format of phone numbers in Mac Contacts app

I saw a lot of questions in the discussions asking how this can be done, but Apple seems to block replies a very short time after a question is asked (seems like only a few months). This seems terribly misguided. Answers are absolutely still relevant after only a few months and even years in some cases. There are so many people here with questions that won't get an answer because of this. Anyway, I would have left this as a reply to one of these questions, but because of this, I couldn't.

This requires disabling SIP on El Capitan or later, but it can be re-enabled after you finish editing the file.

After disabling SIP and restarting, copy

/System/Library/PrivateFrameworks/PhoneNumbers.framework/Versions/A/Resources/Default.phoneformat

to a backup file using

sudo

. (It's easy to mess this up and you may need to revert back to the original at some point.) Open

Default.phoneformat

with a hex editor, (xcode works if the file is opened in hex mode). Find

(###) ###-####

. Delete the parentheses. Be sure to add nulls (

00

) to the end of the file for any characters you remove because Contacts may crash due to hex file size issues. Search for any other phone number formats you would like to change at your discretion. For my purposes (omitting parentheses from US phone numbers in the Contacts app, I removed the parentheses from the two instances of

(###) ###-####

around line

55670

. Save your file and reopen Contacts. You should now see phone numbers in the changed format.

Keep in mind that this file could be overwritten at any time during an update, so you may want to keep a backup of your edits somewhere.