if (response.getState() === "SUCCESS"){
  var result = response.getReturnValue();
     for(var i=0; i<result.length;i++) {
       var newPhoneNo = helper.formatPhoneNumber(component,result[i].Phone__c);
     result[i].Phone__c = newPhoneNo;
      }
  }

formatPhoneNumber: function(component, phone) {
    var s2 = (""+phone).replace(/D/g, '');
    var m = s2.match(/^(d{3})(d{3})(d{4})$/);
    return (!m) ? null : "(" + m[1] + ") " + m[2] + "-" + m[3];
},