Hello @mbrodeur,
Please take a look at SIP Caller, a cloud based outbound call dialer that can be easily integrated with FreePBX, and will do what you need.
SIP Caller connects as a remote extension to your system, and then makes the calls that you need. You can easily define a call flow for appointment reminder (there is a template for that ready to be used), create a campaign, and upload the numbers to call, so SIP Caller starts calling your customers and collecting their input (if they want to confirm or reschedule the appointment).
The dialer will provide real time and historical reports, so you know who confirmed and who didn’t.
We provide a free version which you can use with a limited number of calls per day, or very affordable plans when you need a higher volume.
Please feel free to contact me in case of needing help with this.
Kind regards.
Ernesto Dos Santos Afonso
SIP Caller - Co-Founder
https://sipcaller.com