The written part depends a little bit on the speaker, actually.
If it were a woman, it would be "je suis désolée", but for a man "je suis désolé".
You can also say "pardon" and that works for both genders.
I would personally translate "I can't hear you" as: "J'ai du mal à vous entendre" (similar to "I'm struggling to hear you"), although both answers above are fine.
And the last sentence "We must have a bad line" would be more faithfully translated as "La ligne doit être mauvaise".
-"doit"= must (expressses possibility, in this context)