No need for custom tool call, create a Server URL (endpoint to accept VAPI requests). Set the inbound assistant to blank for the relevant number on the dashboard. Your endpoint will receive an 'assistant-request' when an inbound call comes in - this request contains metadata including the phone number of the person calling in etc. Based on that you can send the assistant JSON back along with custom dynamic values in the prompt and other parameters (such as the name of the customer by checking the phone number against your DB, or 'good afternoon/good morning/good evening' greeting based on timezone). To get the assistant config (JSON), first setup your assistant on dashboard, copy the ID and then use the get assistant endpoint.