Before I was trained in tech support, I would answer a dead phone line the way a normal person would: by giving my standard greeting, and then saying "hello? hello? is there anyone there? hello?" and mumbling broken phrases in increasing volume until finally realizing that there is no one there, and hanging up the telephone. Of course, speaking louder doesn't help the connection, unless the people on the other end happen to be foreign.
When you are speaking on a seemingly dead line, there is a chance that the people on the other end can hear you. I don't know how much of a chance, but it is large enough that in technical support training we were told the following formula to deal with it:
Hello, you have reached XYZ Organization, this is My Name, how may I help you? (This is the standard greeting. Here, you pause for several seconds, because the caller may simply have been on hold too long and be a few feet away from the phone, then repeat:) Hello, you have reached XYZ Organization, this is My Name, how may I help you? (Pause again, briefly.) If there is someone on the other end of this phone line, and you can hear me, I can not hear you. Please call back, perhaps on a different phone line. Thank you.
Such a clear statement, even though it may sound silly to the speaker, makes perfect sense when you imagine how much better it sounds for the (potential) listener, rather than a series of confused mumbles ending with a click. Of course, this can also be used on phone calls with a great amount of static, or any other circumstances where there may be a possibility that the caller on the other end can hear you.