By default, the extension only supports U.S. phone numbers, however users can add custom matching Regular Expression. I recommend testing your regex at https://regex101.com/. When formatting the links, the groups are filled in using {#}. If the regex test showing matching group 1 is 67, then {1} will be replaced with 67. {0} will always replace with the originally matched string.

Update 2/2/2019: Bug fix submitted by carlm0101 via github. Notes:

* Create "call" exactly once at Install time. This eliminates the harmless error message (when "Collect errors" is ON) about a duplicate context menu item.
* In the case where phoneNumberRegex.test(selectedText) fails to find a match, run encodeURIComponent() on selectedText. Solves problem with leading '+' (a special character) in selectedText being passed in URL without encoding and thus not appearing in REQUEST parameter in destination script.

Added icon for browser's menu bar.