Make multiple calls(tasks)

You could use a queue and a semaphore with 4 concurrent entries to implement this. Below is a simple console application that adds 50 phone numbers to a Queue<string> and then process them all by iterating the through the queue. Only 4 threads at a time can execute the code below the call to the WaitOne() method in the MakeCall method:

using System;
using System.Collections.Generic;
using System.Threading;
using System.Threading.Tasks;

namespace ConsoleApplication5
{
    public class Program
    {
        static readonly Queue<string> phoneNumbers = new Queue<string>();
        static readonly Semaphore semaphore = new Semaphore(4, 4); //A semaphore that can satisfy up to four concurrent requests. 
        public static void Main()
        {
            // Populate the queue, i.e. add all phone numbers
            for (int i = 0; i < 50; i++)
                phoneNumbers.Enqueue(i.ToString());

            // iterate through all phone numbers and make the calls
           List<Task> tasks = new List<Task>();

           do
           {
               string phoneNumber = phoneNumbers.Dequeue();
               try
               {
                   //MakeCall(phoneNumber);
                   tasks.Add(Task.Factory.StartNew(() => { MakeCall(phoneNumber); }));
               }
               catch (Exception)
               {
                   //put the number back in the queue if the MakeCall method throws an exception...
                   phoneNumbers.Enqueue(phoneNumber);
               }
           }
           while (phoneNumbers.Count > 0);

           Task.WaitAll(tasks.ToArray());
           semaphore.Dispose();
        }

 private static void MakeCall(string phoneNumber)
        {
            semaphore.WaitOne();

            //call your server here...
            Console.WriteLine(string.Format("calls {0} at {1}", phoneNumber, DateTime.Now.ToString("hh:mm:ss")));
            System.Threading.Thread.Sleep(2000); //simulate that the call takes 2 seconds to complete...

            semaphore.Release();
        }
    }
}