--- phone.module
+++ phone.module.patched
@@ -821,7 +821,7 @@
     '#columns' => $element['#columns'],
     '#attributes' => array('class' => 'phone'),
     '#size' => 17,
-    '#maxlength' => 16, // International standard - allow for the plus sign and 15 digits plus separators - this may not be enough for numbers with extensions    
+    '#maxlength' => 19, // International standard is 16 - allow for the plus sign and 15 digits plus separators - set to 19 to accomodate numbers with extensions    
   );
 
   if (!empty($field['max_length'])) { // TODO Appears to be a non-existent property name.