“I'm sorry, you're breaking up.”

You're talking on your mobile phone to your boss. He is saying something, but you can't hear what it is because the reception on your phone is bad. You didn't hear some of what he said, so you say:

I'm sorry, you're breaking up.

you're breaking up

This is a phrase you use when part of a conversation over mobile phone gets cut off because of bad reception.

You can also say that "the connection is breaking up".