Getting "A From phone number is required" in JSON call to Twilio

Question

I'm trying to use JSON over HTTP to communicate with the Twilio API (I'm running on asp.net vNext - and their C# library doesn't yet support coreclr)

I've got the following code, but every time I call the API I get the same 400 - Bad request response:

{"code": 21603, "message": "A 'From' phone number is required.", "more_info": "https://www.twilio.com/docs/errors/21603", "status": 400}

I'm using the following code to setup the request and make the call - the auth succeeds and I get the above error:

var byteArray = Encoding.ASCII.GetBytes("{AccountSid}:{AuthToken}");

HttpClient client = new HttpClient();
client.DefaultRequestHeaders.Authorization = new AuthenticationHeaderValue("Basic", Convert.ToBase64String(byteArray));

var jsonStr = JsonConvert.SerializeObject(new {
    From = WebUtility.UrlEncode("+15005550003"), // A test number twilio supply
    To = WebUtility.UrlEncode("{my phone number}"),
    Body = WebUtility.UrlEncode("Hi there from a pure json rest client...")
});

var response = await client.PostAsync("https://api.twilio.com/2010-04-01/Accounts/{AccountSid}/SMS/Messages.json", new StringContent(jsonStr));

Answer 1

After contacting support and dealing with a very helpful guy called Joshua, we have figured it out.

In Twilio docs you can either use a url with "messages.json" or "messages.xml" - I took that to mean: whichever url you use, the message (request and response) will have to follow that format.

Unfortunately that is not true - all requests to Twilio, must be of Content-Type: application/x-www-form-urlencoded

e.g. To=0123456789&From=9876543210&Body=Hi

in C# (HttpClient) that looks like:

_client.PostAsync(_url, new FormUrlEncodedContent(new Dictionary<string, string>(){
 { "To", "0123456789" }, 
 { "From", "9876543210" }, 
 { "Body", "Hi" } }));