Hi James! You mean a function where patients are reminded of future appointments? If that is the case, yes, this is possible to do within DHIS2 as long as you have an SMS gateway configured. More information about SMS gateways here: Home - DHIS2 Documentation
For example, the DHIS2 Covid vaccine package has a notification configured with an SMS alert seven days before an appointment is due with the SMS “Dear NAME OF CLIENT, you have an appointment to receive your COVID-19 vaccine on the DUE DATE at NAME OF FACILITY
This is configured in the “Notifications” section within the program configuration:
Does this answer your question? let me know if anything is unclear!