When you do not understand what was said
I’m sorry. I missed that.
Missed that = failed to hear what was said
I didn’t quite get that.
to get that = to hear or understand what was said
What did you say?
I’m sorry. What?
What was that?
Come again. (folksy)
Huh? (informal or rude)
Could you please repeat yourself?
Could you please repeat that?