My test step is continually running without any results. Or, you're getting a "Problem creating Webhook Subscription" error.
In order to use Calendly with Zapier you must have a premium/paid Calendly account, as noted here: https://help.calendly.com/hc/en-us/articles/235289507-Zapier
I need to trigger on rescheduled appointments
Use the Invitee Created or Rescheduled trigger. In the trigger's Invitee type field, select whether the Zap runs for newly scheduled invitees, rescheduled invitees, or both. Leave the field blank to run for both.
When an invitee reschedules, Calendly cancels the original booking and creates a new one, so the Invitee Canceled trigger also runs for the original booking. Keep this in mind if you have a separate Zap that acts on cancellations.