Sending SMS messages is an optional feature in Caterbook, and to use it you must sign up for a paid monthly subscription plan.
SMS plans have an included number of SMS credits per month at a fixed price, and a per message overage fee if you exceed the inclusive credits.
We even have a Pay as you Go plan with no minimum monthly fee.
Bear in mind that some longer messages over 160 characters may require more than one credit to send.
Please raise a support ticket if you'd like to discuss SMS pricing options with one of the team.