I am constantly amazed by how simple some things are, yet how many people are bad at.
I have voice mail. Anyone who has ever listened to a recorded message has probably realized that what you hear on the phone is not exactly CD quality. Plus, you can't see a person, so you can't see some of the important lip cues that we normally use to help make sense of the sound we're making.
Yet people read off their phone numbers really, really fast. And only once! Yes, I can replay it, but if I can't make it out, I am probably not going to be able to make it out on the rewind.
Also had someone phoning who didn't give their last name... Hm.
The moral of the story is: Say it slow, and say it again!