Great question — answering as the UphillTech team:
Make sure you associate the Twilio number your flow is using as the From (outbound CallerID) with the this Studio flow (Under Messaging > A Message Comes In), so when someone sends a response, it gets sent back to your Studio Flow. You do this here, https://www.twilio.com/console/phone-numbers/incoming , when logged into your Twilio console. Studio is a Generally Available (GA) Product, no longer Beta. Also, the execution logs for the Studio flow are useful for providing additional insight into flow behavior.
Full answer by Alan: https://stackoverflow.stackexchange.com/a/56514500 (CC BY-SA 4.0).
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