Trouble getting incoming calls from regular number to twilio number

Question

When I call from my local number to twilio number, the webhook makes a POST request /place-call and then ends with fall back error url.

Twilio is unable to process the Content-Type of the provided URL. Please see Twilio's documentation on accepted content types for more information on valid Content-Types. You must return a Content-Type for all requests. Requests without a Content-Type will appear in the Debugger as a 502 Bad Gateway error.

place-call.js file

const callerNumber = '+1XXXXXXXXXX';
const callerId = 'client:alice';
const defaultIdentity = 'alice';

exports.handler = function (context, event, callback) {
  var url = 'https://' + context.DOMAIN_NAME + '/incoming';

  const client = context.getTwilioClient();

  var to = event.to;
  if (!to) {
    to = event.To;
    if (!to) {
      console.error("Could not find someone to call");
      to = null;
    }
  }
  //  console.log("To: " + JSON.stringify(event));

  if (!to) {
    client.calls.create({
      url: url,
      to: 'client:' + defaultIdentity,
      from: callerId,
    }, function (err, result) {
      // End our function
      if (err) {
        callback(err, null);
      } else {
        callback(null, result);
      }
    });
  } else if (isNumber(to)) {
    console.log("Calling number:" + to);
    client.calls.create({
      url: url,
      to: to,
      from: callerNumber,

    }, function (err, result) {
      // End our function
      if (err) {
        console.log("Error: " + err);
        callback(err, null);
      } else {
        callback(null, result);
      }
    });
  } else {
    client.calls.create({
      url: url,
      to: 'client:' + to,
      from: callerId,
    }, function (err, result) {
      // End our function
      if (err) {
        callback(err, null);
      } else {
        callback(null, result);
      }
    });
  }
};

function isNumber(to) {
  if (to.length == 1) {
    if (!isNaN(to)) {
      console.log("It is a 1 digit long number" + to);
      return true;
    }
  } else if (String(to).charAt(0) == '+') {
    number = to.substring(1);
    if (!isNaN(number)) {
      console.log("It is a number " + to);
      return true;
    };
  } else {
    if (!isNaN(to)) {
      console.log("It is a number " + to);
      return true;
    }
  }
  console.log("not a number");
  return false;
}

This is the error from the error log:

Twilio is unable to process the Content-Type of the provided URL. Please see Twilio's documentation on accepted content types for more information on valid Content-Types. You must return a Content-Type for all requests. Requests without a Content-Type will appear in the Debugger as a 502 Bad Gateway error.

incoming.js file

//'https://' + context.DOMAIN_NAME + '/incoming'

exports.handler = function(context, event, callback) {
  const twiml = new Twilio.twiml.VoiceResponse();
  twiml.say("Congratulations! You have received your first inbound call! Good bye.");

  callback(null, twiml.toString());
};