Free $5 Digital Gift Card for Xbox Live when you spend $15 or more on Xbox Live and upload your receipt to your phone by texting "XBOX LIVE" to 30364.
Follow the Best Buy link here for full details.
UPDATE: This appears to be part of a larger Xbox promotion called 'Get the Most With Xbox Live'. The free gift card part of the promotion spans multiple retailers, not just Best Buy.
Full promotional details from Xbox here.