To use the SMS features available in YesBookit you need to do the following:
Go to http://www.smsglobal.com and sign up.
Buy SMS credits
In YesBookit go to Options > Settings > General tab
Enter the office mobile phone number in the SMS Number field
Enter the SMS Global Username and password in the appropriate fields - see here for how to access those details
Optionally enter a name in the From field to hide the phone number the SMS is sent from
Once you have done that you can then optionally use the SMS functions under Options > Settings > Bookings tab ie
Bookings SMS
Requests SMS
SMS Reminders
Departure SMS
There is no charge from YesBookit to use the SMS features.