Justoverclock one way as I’m sure you already thought about, is adding this to hashtags. No need for a separate new extension. Just pick a character that should be used in front of the number, and duplicate/slightly modify the hashtag RegEx.
So maybe, to auto link a phone number you must first type +? That way the RegEx can look for any number preceded by +.
The only downside, just like a BBcode, your users have to know and remember to do it like this.