Buttons to decline call for reason

I received a call on my iPhone 3gs; when I pulled it out of my pocket I saw that the screen had a list of reasons, such as "I'm on my way," "I'm busy right now", and maybe three others. But I was in a meeting and out of habit quickly pressed the wake/sleep button twice to decline. The reason buttons disappeared. The caller was a loan company with whome I'm behind on payments.

I've never seen those before. And because they are useless for a traditional voice telephone, I have one theory that it's a feature available only between iPhones. BUT, on the other hand, I have had a innumerable number of voice calls from friends with iPhones and have never seen this.

Anyone know about these buttons?