You only need to respond with TwiML to a webhook from Twilio. In this case it appears that you are responding with TwiML to your application's front end when a submit button is pressed.
So, you can drop all the TwiML:
@app.route('/', methods=["GET", "POST"])
def home():
form = ConferenceList() #form made using flask-wtf
if form.validate_on_submit():
contact_1 = form.data['contact_1'] #callee1
contact_2 = form.data['contact_2'] #callee2
'''here I feel could be a bottleneck'''
add_user(contact_1, conference_name='Conf', label='laed#1')
add_user(contact_2, conference_name='Conf', label="consumer")
return "whatever"
Since the settings you are trying to apply in the TwiML do not apply to the participants joining the conference, you need to adjust your add_user function to use them. In this case, the startConferenceOnEnter and endConferenceOnExit parameters for the moderator should be sent when you create the participant. It might be easier to write two methods, add_user and add_moderator, to make things clear:
def add_user(contact, conference_name, label):
participant = client.conferences(conference_name).\
participants.create(
label=label, #label for participant
beep='onEnter',
record=True,
from_='from_', #same as above
to=str(contact),
start_conference_on_enter=False
)
def add_moderator(contact, conference_name, label):
participant = client.conferences(conference_name).\
participants.create(
label=label,
beep='onEnter',
record=True,
from_='from_',
to=str(contact),
start_conference_on_enter=True,
end_conference_on_exit=True
)
Then call different functions for the different participants:
add_moderator(contact_1, conference_name='Conf', label='laed#1')
add_user(contact_2, conference_name='Conf', label="consumer")
When you call add_user or add_moderator it will make an API request and slow down your server response. If you wanted to offload those requests to a worker, that would make your response quicker. But for 2 API requests, it is likely not a problem.
One other thing you might want to consider is the consumer experience. If they answer the phone before your agent does, then they will be greeted with hold music. You might want to architect it so that the application calls the agent first and only once they have picked up it then dials the consumer. Just worth considering.
Edit
After further explanation, you are now telling me that you want to make the call from the browser using the Twilio Voice SDK for JS.
To make outbound calls with the JS SDK you need to create an access token which includes an outgoing application sid, which refers to a TwiML application. That TwiML application has a voice URL. When you place the call with the SDK, Twilio makes a webhook request to the voice URL of your TwiML app. Your application can perform actions and return TwiML to tell Twilio what to do with the call.
When you create the call with the JS SDK you can pass parameters to the call.
const device = new Device(token);
const call = await device.connect({
params: {
To: ["+15551234567", "+145557654321"]
}
});
Those parameters are sent with the webhook request to your TwiML App voice URL. You can then use a response very similar to your original code to respond here, because you need to return TwiML to the request from Twilio, and start calls to the other participants in the call.
@app.route('/conference', methods=["POST"])
def conference():
const numbers = request.form["To"]
response = VoiceResponse()
with Dial() as dial:
dial.conference(
'Conf',
start_conference_on_enter=True,
end_conference_on_exit=True
)
response.append(dial)
for number in numbers:
add_user(number, conference_name='Conf')
return Response(str(response), 'text/xml')
This code receives the To parameter, a list of numbers to dial into this conference, builds the TwiML response that will put the browser caller into a conference call, places outbound calls to the numbers to dial them into the conference and then returns the TwiML to Twilio. The dialler in the browser will start the conference and the other participants will arrive in the conference when they answer the phone.
In this case you don't make the request to your server yourself, you use the JS SDK to trigger the call and let Twilio make the request to your server. As mentioned in the comments, you may want to offload the API calls to create participants to a background job so that you can respond to the webhook request quicker, but that is beyond the scope of this answer.
