Why strip only the extension, this can be done by the user

actionpack/lib/action_view/helpers/number_helper.rb

@@ -60,7 +60,7 @@ def number_to_phone(number, options = {})
         options      = options.symbolize_keys
         area_code    = options[:area_code]
         delimiter    = options[:delimiter] || "-"
-        extension    = options[:extension].to_s.strip
+        extension    = options[:extension]
         country_code = options[:country_code]
 
         str = ""