Sorry to disappoint you, but if it comes down to actual hearing on a bad line, your English still needs work. A native speaker uses his or her knowledge of the language to decipher what is heard, even on a bad line. This is called "top-down" perception, in which the brain leads the ears, rather than the other way around.
Beginners and intermediate students don't have enough familiarity with the sounds and usage of English to do this. So, a bad line will make it difficult for them.