I seem to be on a roll with phone etiquette so I will continue on with that for a bit.
The Well Mannered Adult...
...Checks their voicemail FIRST before calling someone back.
Does the following scenario describe you?
You remember you have to call your doctor's office because you've got 3-4 questions in follow up to your last visit. You call their office, which is always busy and it's hard to reach a live person. You leave a message with a rundown of your questions and hang up. A little while later you check your phone and realize someone from that office left you a message. Instead of LISTENING to the message, you call back saying something like: "Hi, this is Jane Doe. Someone from your office tried to call me?"
Can I just tell you now how annoying that is? You remember when you were complaining that you could never reach a live person? One of the reasons you can never reach a live person is because the staff at the office is busying on the phone repeating the EXACT SAME INFORMATION THEY ALREADY LEFT ON YOUR VOICEMAIL.
We're all busy. I get it. But if you're too busy to listen to the response to your inquiry the first time, then you're too busy to make the inquiry to begin with.
Extend courtesy to others and value both their time and your own. Pay attention the first time.