Got the same message about a $100 gift card. The Text came very early, waking me up!!! I had just placed an order on a site the previous day, but after reading these comments, I know it's not related.
Text message from this number: Thanks 4 visiting our website. Call 800-571-4835 to claim you $200 Walmart voucher. Reply STOP to unsub. Did not call and did not reply. Don't know who they are or how they got my number.