Do I have to provide a opt-out option on every every SMS I send?
This is not required to be GDPR complient, but may be recommended to have it on. The requirement is that everyone must have access to an opt-out posibility, and customers have this in their member's area.
In the case of marketing via SMS, there should be an option to report via SMS, as well as via the trader's website.
In Diller, we have set up an optional service for GBP 5 a month if you still want to add the unsubscribe option to every SMS sent. Contact our support team at support@diller.io for help setting this up for your loyalty club.