The button will answer a phone call or hang it up with one press, or reject it with two. You can also use voice commands "Answer" or "Ignore" (assuming your phone is compatible). If you take the ear bud out mid-conversation, the call will immediately transfer to your phone (nice not to have to remember to press a button to do that!) You can also set it to NOT transfer a conversation immediately back to the phone, if you're someone who likes to take your ear bud out a bit now and then during a phone conversation. And you can use voice commands through the Left ear bud, too (again, all phone conversations are heard through the bud labeled "Left," though you can move it to your right ear). So, there is a Left and a Right ear bud. The Left is the one that actually pairs with your phone, while the Right bud just pairs with the Left bud.