Using Angularjs $save method with .Net Web Api

I am using Angularjs $Resource $save CRUD method to submit data to RESTFull .Net Web API webservice:

phone.$save({}, function (data) {
        alert('success');
    }, function (data) {
        alert('failed');
    });

Where phone is a $resource object. The call successfully passes the phone object to the Web API:

public async Task<Phone> Post([FromBody]Phone phone)
    {
        try
        {
            if (ModelState.IsValid)
            {
                phone.CreatedDate = DateTime.Now;
                this.Uow.Phones.Add(phone);
                this.Uow.Commit();

                return phone;
            }     
        }
        catch (Exception ex)
        {               
            throw new HttpResponseException(new HttpResponseMessage(HttpStatusCode.InternalServerError));

        }
        return null;
    }

but when returning the response the failed callback is triggered with the following error:

"The 'ObjectContent`1' type failed to serialize the response body for content type 'application/json; charset=utf-8'."

In Angularjs documentation for the $save function we have:

card.$save();
// POST: /user/123/card/456 {id:456, number:'1234', name:'J. Smith'}
// server returns: {id:456, number:'1234', name: 'J. Smith'};

Which means that the response should return the same object but with updated fields.

How can a valid response be returned from the Web Api?

Thanks!