public static void ValidatePhoneNumber(this MpesaClient mpesaClient, string phoneNummber)
 {
     LengthValidator.ValidateLength(phoneNummber, "PhoneNumber", 12);
     if (!phoneNummber.StartsWith("2547"))
     {
         throw new Exception("Invalid phone number");
     }
 }
 public static void ValidateBusinessShortCode(this MpesaClient mpesaClient, string code)
 {
     NumericValidator.ValidateIsNumeric(code, "BusinessShortCode");
     LengthValidator.ValidateLength(code, "BusinessShortCode", 13, 5);
 }