I have set up as I was prompted by customer service (who reply once per day by the way, very poor support) the following settings :

  Message API access
Message provider :
Build-in Mail [MAIL] select provider type for notification
Mail subject :
Reminder Fields like Phone field, Send from, Country code will be ignored for email. Phone field :
E-mail field that will be used as phone number for sending SMS to. Customer number Send from : phone number that will be used. Some providers allow alphanumeric values as sender Country number : you can allow customer to enter numbers without country part. This part will be added before number that customer entered. NOTE: leading 0 will be removed. If you don't want to use it leave it blank. Message Template :
Σε εκκρεμότητα Κράτηση Canceled Επιβεβαιωμένο Reminder Follow up Σας υπενθιμίζουμε το επιβεβαιωμένο ραντεβού σας στις #date# και ώρα #start#. Σας περιμένουμε με χαρά στο Μπαρμπεράκι !
#
Tags : #id#, #date#, #start#, #end#, #status#, #created#, #price#, #ip#, #link_confirm#, #link_cancel#, #url_confirm#, #url_cancel#, #service_name#, #service_duration#, #service_price#, #worker_name#, #worker_email#, #worker_phone#, #location_name#, #location_address#, #location_location#
Custom fields : #email#, #name#, #phone# Send to :
Customer select who will get notification Send message reminder :
3 hours send same message notification before appointment
Follow Up Message :
off send Follow Up notification after start of appointment plus selected offset
limit message per Month :
0 limit number of messages per month. Set to 0 or without any value to have no limit. Current Message count for this month is: 0


But my customers do not receive any reminders. 
This is very rough, as I am a PRO user for years, and I do not seem to be getting any type of help