To trigger an action, reply with a JSON struct to dictate what should happen:
// The ”forward” action
Static control flow in JSON
If you can't, or don't want to, run your own backend there is limited support
for handling control flow by storing a JSON struct as the sms_url.
However, we strongly recommend that you use a webhook
and avoid putting control flow directly in a static JSON struct if at all
possible. It's really easy to make a mess.