When a customer has a phone number only, they will receive the offer by SMS, when they have an email address only, they will receive the offer by email. When a customer has both email and SMS in the CRM record, they will receive links to the offer by both email and phone, but it is the same single  offer sent by both modes. Redeeming the one offer (email or SMS) will invalidate duplicate redemption of the offer. However, if a customer unsubscribes to the offer the "Marketing Permission" flag is turned off and  no further offers will be sent, unless the flag is manually turned back on again.