I have a very simple flow based off the appointment reminder template that simply sends a reminder, waits for a reply, then based on a user response, sends an appropriate response to the user. I removed webhook widgets as this was just a test to see if the flow would keep state and respond, as expected.
I have configured my phone number to send incoming SMS messages to this flow: Messaging routing for active phone number in Twilio
Additionally, the service specified simply states for messages to be handled by the sender's webhook: Service webhook config
When I initiate the flow via a REST API call, all starts just fine. My phone gets the message. In the flow log, I see the flow is now active.
When I reply to the message, I see my reply in the logs for the flow in question. However, I receive no reply from the flow. Twilio Flow Execution Log showing a REST API invocation and a response. The response does not result in action within the flow as expected.
If, however, I hook up the "Incoming Message" trigger from within the flow to one of the widgets, it executes, again, demonstrating that my messages are indeed getting routed in the general direction. (Of course, this initiates a new flow and does not resume from the state in the flow instance created by the REST API call.)
I've seen other Stack Overflow queries on similar issues. The screenshots I've included are the result of my following those suggestions to ensure that incoming messages are indeed routed to my flow. Yet, I can't seem to get the flow to respond how I would expect.
It seems I've overlooked something but, based on what I have seen in Twilio tutorials and other questions here, I can't for the life of me figure out what it is.
I'd recommend removing the messaging service from your scenario. The way I see it, it's not required and you can achieve the same result by connecting the phone number to the Studio flow.
PS: If you don't want to delete the service, it's also fine if you just remove the phone number from the sender pool of the service: