As a call ended, the closing message could sound stretched or distorted right at the very last word, rather than finishing cleanly. This happened because the line closed a touch too soon after the goodbye was spoken — based on an estimate of how long it takes to say, since the phone network gives us no way to know for certain when speaking has actually finished.
We've given noticeably more time before the line closes, and it no longer closes in the same instant the goodbye is sent. No call was ever cut short or dropped early because of this — it only affected the sound of the very last moment of a call.