question

1. Is it possible to find a user/extension by searching by a custom field? We are using a custom field to store our company unique assigned ID so t hat we can, hopefully, tie a user/ext in RC to our internal employee ID.

As far as I know, there is no custom field search filter.

2. Is it possible to add sites/companies into the sandbox? Our company uses sites in production and I would like to test various things in an environment that can more closely mimic prod.

Yes, you can open a support ticket and request for this.

3. Do you have a demo of the API events/notifications system or a sample response that would be sent from that system?

What events/notifications?

4. The API site for creating an extension doesn’t show what fields are absolutely required. Do you have a list?

The minimum requirement is a contact object and the type

var params =
      contact:
        firstName: "
        lastName: "
        email: "
        emailAsLoginName: true/fal
      
      type: "User"
    }

the notification I was most interested in was one that would show extension numbers and/or a phone number being changed.

You can add this class to your code. Remember to install the latest RC PubNub SDK.

static private async Task pubnub_notification()
{
    try
    {
        var eventFilters = new[]
                {
                    "/restapi/v1.0/account/~/extension/~"
                };

        var subscription = new Subscription(rcsdk, eventFilters, message =>
                {
                    Console.WriteLine("Incoming event");
                    var jsonObj = JObject.Parse(message);
                    Console.WriteLine(jsonObj["body"]);
                });
        var subscriptionInfo = await subscription.Subscribe();

        Console.WriteLine(subscriptionInfo.id);
        Console.WriteLine("Subscribed");
        while (true)
        {
            Console.WriteLine("looping ...");
            Thread.Sleep(5000);
        }
    }
    catch (Exception ex)
    {
        Console.WriteLine(ex);
    }
} 
// sample event data

{  
    "extensionId": "17800xxxx
    "eventType": "Update"
    "hints":[
        "AnsweringRules"
   ]
}

You will get just the event. There will be no info about the change though.

Once the ability to add sites in sandbox is active, how do you add one?