When you create an agent, it is assigned a random phone number. If someone calls that number, the agent will answer with a default greeting, and present several options to the caller, such as "book a call" or "leave a message". If the caller says one of those options, the call will be directed down the corresponding flow. For example, if the caller says "leave a message", the call will go down the "leave a message" flow, asking for their name and message, and then redirecting them back to the main menu once the flow is complete.
If the caller says something else that is not one of the flows, the utterance will be passed to OpenAI's GPT-3, which is pre-prompted with the information about your business that was supplied when the agent was created.
Ultimately, these initial options, and whether or not to use GPT-3 as a fallback, is all customizable. You can submit a "change request" in your dashboard to make changes to, additions to, or remove any of these things.