public string TestCalls()
        {
            StringBuilder testResult = new StringBuilder();

            HTCOneM8.AddCall(new Call(new DateTime(2015, 03, 06), new ContactInformation("Nikolay", "+359 882"), 366));
            HTCOneM8.AddCall(new Call(new DateTime(2015, 03, 05), new ContactInformation("Marya", "+359 883"), 101));
            HTCOneM8.AddCall(new Call(new DateTime(2015, 03, 04), new ContactInformation("Lilly", "+359 884"), 244));
            HTCOneM8.AddCall(new Call(new DateTime(2015, 03, 06), new ContactInformation("Nikolay", "+359 882"), 12));

            testResult.AppendLine(String.Format("Adding calls: {0}", TestAddingCalls()));
            testResult.AppendLine(String.Format("\t\tTotal call price: {0:C}", HTCOneM8.GetTotalCallPrice(0.37m)));
            testResult.AppendLine(String.Format("Find longest call: {0}", TestLongestCall()));
            testResult.AppendLine(String.Format("Remove longest call: {0}", TestRemovingLongestCall()));
            testResult.AppendLine(String.Format("\t\tTotal call price: {0:C}", HTCOneM8.GetTotalCallPrice(0.37m)));
            testResult.AppendLine(String.Format("Remove all calls: {0}", TestRemoveAllCall()));
            testResult.AppendLine(String.Format("\t\tTotal call price: {0:C}", HTCOneM8.GetTotalCallPrice(0.37m)));

            return(testResult.ToString());
        }