Voice Agent Booking Over Busy Times Even Though They're Marked as Busy

Hey everyone, I’m building an inbound AI voice agent using Vapi, Make.com, and Google Calendar. The agent acts as a receptionist for pool contractors, qualifying leads and offering available times to book a strategy call with our team.

Right now, I’m running into a major issue where the agent is offering and booking times that are already marked as busy on the calendar.

The Core Issues

1. Agent is Booking Over Busy Times

In Make.com, I’m using the Google Calendar "Get Free/Busy Information" module. It correctly returns busy blocks, and I can see those times are marked as unavailable in the output.

Despite this, the agent still suggests and confirms appointments during those busy times. For example, if my calendar is busy from 12 AM to 2 PM on May 30, the agent will still say “You’re available at 11 AM” and book that time.

2. Returned Time Zone is Incorrect

The times returned by the agent seem to be in UTC, but the agent says them as if they’re in Eastern Time. This causes the agent to say things like “1 PM Eastern” when it’s actually several hours off.

I think the time values need to be formatted in Eastern Time before being spoken.

3. {calendar_availability} Tool Is Not Filtering Out Busy Times

The calendar module returns the correct busy periods, but I don’t think my Make scenario is comparing those to the time options being passed back to Vapi. Right now the webhook response just includes available options, but it doesn’t actually exclude any overlaps with the busy[] array returned by Google Calendar.

Screenshot

Here’s a screenshot showing Google Calendar correctly marking the time from 12 AM to 2 PM as busy. But the agent still sees that time as available:

What I’m Hoping to Solve

How can I properly filter out busy time blocks in Make.com before the agent sees them as available?

What’s the best way to convert UTC time to Eastern so the agent speaks the correct local time?

If anyone has experience with this setup or a working calendar logic they’d be willing to share, I’d really appreciate it. Thanks in advance.

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