You can set up some templates (Quick-codes) that can be used as a 'message' when sending ad-hoc SMS messages from the Appointment Book.

You create Template text as usual in Setup > Templates > Letters and make sure the SMS option is checked on.

Some examples of the text you can use are:

Reporting Time

<first name>, your operation is booked for <ApptDate> at <userfname>.  Please report at <ApptRepTime> on <ApptRepDateFull> to be admitted.

Changed Reporting time

<first name>, the reporting time for your operation  on <ApptDate> at <userfname> has changed to <ApptRepTime> on <ApptRepDateFull>.  Please contact us if you are unable to make this new time.

Fasting From time

<first name>, this is a reminder that you are required to fast from <ApptFast> for your operation on <ApptDate> at <userfname>