Patients can stipulate - usually on a New Patient Form or verbally thereafter - the methods they wish to be contacted via, then this should be set in their file.
Go to Patient records > Patients tab > Address sub-tab
Ensure the patient has a Mobile and/or E-mail entered, depending on how reminders will be sent
Ensure the patient has their preferences selected for Send Appt. Reminders by clicking then selecting preferred methods
If a patient does not want to be reminded of Appointments, select
The Patient has requested no further communication