One number, not a personal one
Once patients have a doctor's personal number, that cannot be undone. Use a clinic number that more than one person can answer and that stays with the practice.
Keep clinical detail out of it
Use it for appointments, reminders, directions and simple follow-up. Reports, diagnoses and prescriptions belong in the record, where they are stored properly and can be found later.
Ask before you send
Sending unsolicited messages to patients is both a privacy problem and a fast route to being blocked. Take consent, and make it easy to opt out.
Templates save more time than they look like they will
The same four or five messages cover most of what a front desk sends. Writing them once, properly, is the difference between a considered message and a rushed one.
In short. Clinic number, no clinical detail, consent first, templates for the rest.