We are a store where our initial contact point is over the phone. During the intake process we always ask
"Would you like to receive text message reminders of when you might be due for your next order".  If they say yes, we mark it in our system which records the date and time.  These are all for medical supplies that are available over the counter but we do have prescriptions.  

  Additionally, when the messages are sent out, customers have the option to reply "stop" to opt out of receiving further notifications.