SMS is a phone message service for reminding people of their appointments -

when you schedule your appointments its put in the doctors computer schedule planner, then the day before your appointment the computer system dials your phone number and a generic message is played to who ever picks up the phone or to your phones answering/voice mail box saying something like -

"this call is to remind amanda that she has an appointment on monday november 29th at 9am, please call ---------- to cancel and reschedule if Amanda cannot make this appointment. thank you, good bye."