tl;dr: add button for phonenumber link support similiar to the mail adress button
Because more and more companies/people use cloud managed phone systems (with 'phone apps') and because mobile devices support it I think it would be nice to have an option added in the WYSIWYG editor for html tel: links. Currently this can only be done through html i.e.
According to html - How to mark-up phone numbers? - Stack Overflow and similiar posts consensus seems to be that tel: markup is the official standard for this.