What do you say when you talk over phone but you can't quite hear what other person says? [closed]

If your intention is to inform the other person that you're having difficulty understanding them because the signal is weak, you could say:

TO FRIENDS/INFORMAL:

Sorry, I didn't catch that. You're fading out.

FORMAL

I'm sorry, but I didn't hear what you said just then. We have a bad connection.

or

I'm sorry, but we have a bad connection and I'm unable to hear you clearly.

All of your suggestions are good.

You can also start off by saying one of:

Sorry. I can't hear you.

Sorry. I didn't catch that.

Then follow it up with any of:

It's a bad line.

The line is bad.

You're breaking up.

You're distorting.

You're cutting out.

There are a lot of other ways of saying the same thing.