AngularJS Example:

HTML

<script src="https://ajax.googleapis.com/ajax/libs/angularjs/1.6.4/angular.min.js"></script>
<form ng-app="myApp" name="signupForm" ng-controller="MyCtrl">
  <div class="form-group col-xs-6">
    <input type="text" ng-model="user.MobileNo" placeholder="Mobile Number" 
    name="mobile" required ng-maxlength="10" maxlength="11" id="default1" 
    ng-change="mobilenoCheckFun(user.MobileNo);">
   <span ng-show="signupForm.mobile.$error">{{serverMessage1}}</span>
   </div>
</form>

JavaScript

var app = angular.module("myApp", []);
app.controller("MyCtrl", function($scope){

	$scope.checkIfNumberExists = function(list, mobileno){
  	if(list.length != null && list.indexOf(mobileno) > -1)
    	return true;
    else 
    	return false;
  }

	$scope.mobilenoCheckFun=function(mobileno)
  {
  		//define an array to check If 'mobileno' exists, in your case this check happens at server side. but the logic is same. This list is fetched from DB in your case
    	var list = ["8123791223", "8123791225", "8123791221", "8123791203"];
      var result = $scope.checkIfNumberExists(list, mobileno);
      if(result)
        $scope.serverMessage1="Mobile Number Exits";
      else
         $scope.serverMessage1="";    
  }
});