Asynchronous handler completed while asynchronous operation was pending - C #

Hello, I have some questions regarding the code below.

The first item, I solved using the call of this method with a Task.Factory.StartNew , since in this case, I do not need the answer of the method to follow but what if this were not the case?

public async void SMSAsync(String mobileNumber)
{

  Guid gui = Guid.NewGuid();

  using (var httpClient = new HttpClient())
  {
    httpClient.DefaultRequestHeaders.TryAddWithoutValidation("authorization", "Basic XPTO");               
    httpClient.DefaultRequestHeaders.TryAddWithoutValidation("accept", "application/json");
    httpClient.DefaultRequestHeaders.TryAddWithoutValidation("Content-Type", "application/json");

    using (var content = new StringContent("{  " +
        "\"sendSmsRequest\": {    " +
        "\"from\": \"WeBlank\",    " +
        "\"to\": \"55" + mobileNumber + "\",    " +
        "\"schedule\": \"2014-08-22T14:55:00\",    " +
        "\"msg\": \"Aqui vai a mensagem de SMS.\",    " +
        "\"callbackOption\": \"NONE\",    " +
        "\"id\": " +
        "\"" + gui.ToString() + "\", "     +
        "\"aggregateId\": \"1111\",    " +
        "\"flashSms\": false  }}", System.Text.Encoding.UTF8, 
        "application/json"))

    {
      using (var response = await httpClient.PostAsync("https://api-rest.zenvia360.com.br/services/send-sms", content))
      {
          string responseData = await response.Content.ReadAsStringAsync();
      }
    }
  }
}

link

How do I call the method

public IHttpActionResult CreateUser(ParticipanteCreateModel participante)
{
   // código que executa a criação do usuário
   String telefone;
   telefone = part.Telefone.Replace('-', ' ');
   telefone = telefone.Replace('(', ' ');
   telefone = telefone.Replace(')', ' ');
   telefone = telefone.Replace(" ", String.Empty);
   Task.Factory.StartNew(() => SMSAsync(telefone)); 

   return Ok(usuarioId);

}