Once you are connected with your patient in your Video appointment, to add another participant, click on the '+ person' icon at the bottom of the screen.
This will initiate the option on your desktop to click on 'Dial phone number (audio Only)'.
Users will see the following screenshot for all iOS devices.
PLEASE NOTE: Inviting a 3rd person by selecting 'Dial phone number (audio Only)' does not expose your personal cell number to either party.