C#: Resource for parameter [accountId] is not found

I'm working on a sandbox account, trying to use the Get Extensions API. My code is as follows:

using System;
using System.Threading.Tasks;
using RingCentral;

namespace Send_SMS
{
    class Program
    {
        const string RINGCENTRAL_CLIENTID = "<sandbox-id>";
        const string RINGCENTRAL_CLIENTSECRET = "<sandbox-secret>";

        const string RINGCENTRAL_USERNAME = "<sandbox-username>";
        const string RINGCENTRAL_PASSWORD = "<sandbox-password>";
        const string RINGCENTRAL_EXTENSION = "101";

        const string RINGCENTRAL_PRESENCE_USER = "<sandbox-username>";
        const string RINGCENTRAL_PRESENCE_EXT = "101";


        static RestClient restClient;

        static void Main(string[] args)
        {

            restClient = new RestClient(RINGCENTRAL_CLIENTID, RINGCENTRAL_CLIENTSECRET, false);
            restClient.Authorize(RINGCENTRAL_USERNAME, RINGCENTRAL_EXTENSION, RINGCENTRAL_PASSWORD).Wait();
            get_presence().Wait();

        }
 
        static private async Task get_presence()
       {
            var response = await restClient.Restapi().Account(RINGCENTRAL_PRESENCE_USER).Extension().List(listExtensionsParameters);

            Console.WriteLine("Extension for Test Account: " + response);
        }

    }
}

Where anything in <> is taken from the https://developers.ringcentral.com/my-account.html#/application-credentials site.

I was able to do the Send_SMS tutorial successfully, but for this, I'm getting:

Content: {

"errorCode" : "InvalidParameter",

"message" : "Resource for parameter [accountId] is not found",

"errors" : [ {

"errorCode" : "CMN-102",

"message" : "Resource for parameter [accountId] is not found",

"parameterName" : "accountId"

} ],

"parameterName" : "accountId"

}

What am I missing here?