If you have not sent out an SMS messages via your CRM account but you can see SMS costs on your monthly bill it is because your user(s) are setting up SMS reminders instead of email reminders in their calendar events.  

Please note: SMS reminders are an additional cost to send which for 1 single message up to 160 characters long is 10p per message. 

If an event reminder is left as the default E-mail option you will not be charged for these email reminders.