There are various ways of conveying difficulty in hearing a caller:
You're breaking up, it's really difficult to hear you
This is a really bad connection, could you please call back
It's really difficult to hear you, could we try again
You sound very spotty, could you call again
Breaking up is a hold over from radio usage days, it can be used for both mobile and landlines.
Usually the caller is asked to call back and hopefully a better connection is made.
When appropriate, the caller may ask if it's possible to recall on a landline, or the callee may offer a landline to call on.
In extreme circumstances
I can't hear you, can you hear me?
is used and after a few moments the call is automatically ended with reattempts by the caller, or by either side if they know each other.