The circle on each bud is a button you can use to navigate music and calls. Here's how to use it.
Play/Pause = Quick press
Next Song = Double press the right earbud. (Note: this only works when the music is playing.)
Answer Call = Quick press
End Call = Double press
Cancel Call = Press and hold for two seconds
Voice Command = Double press (Note: This only works with music is not playing and there's no connection nearby.)
Third Party Calls:
End current call and answer new call = Quick press
Reject new call = Press for one second
Answer new call but hold current call = Double press
Switch between calls = Double press. A tone will sound.