The Booking Calendar does not have the SMS notifications functionality in the core of the plugin.
This plugin “Booking Calendar – Clockwork SMS” : http://projectdmc.org/plugins/booking-sms/ was developed by another company (not by us).
They declare that they have this functionality. Please contact support of that plug-in about the compatibility and usage of this Clockwork plugin.
Please note, its does not our plugin and we do not support this plugin.