为什么在下面的例子中，点击按钮时不会调用$ render？

<input type="text" ng-model="client.phoneNumber" phone-number>
<button ng-click="client.phoneNumber='1111111111'">Change phone number</button>

.directive("phoneNumber",function() {
  return {
    restrict: 'A',require: 'ngModel',link: function(scope,element,attrs,ngModel) {
      ngModel.$render = function() {
        alert('rendering');   // This is not called
      };
    } 
  };
});