On the back of Burger King receipts, there is a phone number to call to take a survey. This phone number should be on the back of the receipt for anything you purchase at Burger King.
The survey asks for your opinion on restaurant cleanliness, the staff, how you liked you food, etc... It should take less than 5 minutes. A touch tone phone is required.
Once you complete the survey, you'll be given a code to write on the back of the receipt. This will give you a free whopper or chicken sandwich with purchase of a medium drink & fries.
This offer can apparently be redeemed once each month.
From the back of the receipt:
One free Original Whopper sandwich with purchase of medium fries and drink after completing our brief survey.
1. Call toll free 1-866-425-4745 anytime within 48 hours
2. Complete survey and get validation code
3. Write code here ________