You are breaking up.
"(Telecommunications) (intr) (of a person making a telephone call) to be inaudible at times, owing to variations in the signal: you're breaking up. "
Free Dictionary
"Jenny, you're breaking up," said Rick.
Urban Dictionary.
In Videos:
In Cartoons:
In Educational Materials:- “You’re breaking up. Could you call me back?”
(breaking up = you can only hear parts of what the other person is saying) EspressoEnglish