Hi,
These kind of things normally can be achieved with Automations, which are part of the Enterprise version. Automations are documented here: https://docs.acymailing.com/main-pages/automations
However, we do not have an integration with Calendly. Therefor you won't find the proper filter in tha Automations to select the subscribers.
You could try to setup a filter in the Automation using the Universal add-on. For this you would need to set an sql query as filter. It's documented here: https://docs.acymailing.com/addons/all-cms-add-ons/universal-filter