If u have an iPhone and your talkin to someone and someone calls you on the other line and u answer it and after the phone call is over and u hang up your phone that number will call u back because u still had someone else on the other line . That's all there's no mystery to it
I also have a Sprint iPhone 4S. I've noticed that if you are on the phone with someone and during the phone conversation you have someone else call in and you put the 1st person on hold to answer the 2nd call and then you finish the phone conversation with the 2nd person and then you hang up (because in my case I keep forgetting I had someone else I was originally talking to and NOT because I'm rude) the phone instead of switching back to the first person your phone will automatically call you back with the 865-6696 and the first person you were talking to will STILL be on the phone unaware that you had hung up in the first place. Don't believe me? Then try it out. :)