Asynchronous handler completed while asynchronous operation was pending-C#

Hello, I have some doubts regarding the code below.

1. the code returns the following error [InvalidOperationException]: asynchronous module or handler completed while asynchronous operation was pending.
2. the main method that calls this task (this other methods) returns an Ihttpactionresult and the response of the SMSAsync method ends up interfering with the response of my main method, i.e. I would like to return status 200 even with error in the SMS method - I have tried try cath without any action in cath ;

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 was 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();
      }
    }
  }
}

https://gist.github.com/carloshenriqueribeiro/0b879e1655e7eedd605e264d50a45742

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);

}