After a few iterations in the feature request, the ability to send SMS is live through an integration with Twilio.
To use the SMS functionality, you need to create a Twilio account and
add credits to it
. Once you have an account, enter your Twilio credentials in the SMS Settings
menu in the add-on.
Remember, these reminders only work for contacts with phone numbers in Google Contacts.
The decision to make the user signup to Twilio allows for i)
ability to use own number
(see guide here), ii) user-owned cost control
. If you prefer to have a credit system instead, please leave the feedback in Feature Requests.