Setting up reminders without paid services

In Admin → Messaging choose for each channel:

Email - sent from CareLink's own mail server, replies go to your clinic email.
WhatsApp - click-to-send: reception sends each reminder from your clinic's own WhatsApp. Free and within WhatsApp's rules.
SMS - your clinic's own Android phone with the free open-source SMS Gateway for Android app and your normal SIM. Check your mobile plan allows business texting.
Patients with the app can also get free notifications.

Prefer a paid provider? Choose Twilio and enter your own account details.