The "Goodbye" or "Please press 3 to stop reattempts" prompts play if the lead's line hangs up first.  

You will hear "Goodbye" if the call will not park to generate reattempts (this is the final call S2L will make for the lead).
You will hear "Please press 3 to stop reattempts" if the call has not met the connected call duration minimum. If you do not press 3, the call will park to generate a reattempt. 

If this happens unexpectedly and in the middle of a conversation, it's possible the call dropped on the lead's end due to connection/bad service issues or battery loss.