As the person who answers 911 (hundreds of times a day), I have a few comments.
When people get a busy signal: that's most likely due to a high visibility incident like a fire or accident on the freeway. Dozens of people call at the exact same time, and some don't get through. Very frustrating for everyone involved. Some cities (like mine) have an overflow system where you will always hear a ring, even if all my lines are in use. As soon as I have a free line it will ring and I'll talk to you. The other possibility is that you are on your cell phone and therefore routed to highway patrol. They have large call centers who take forever to answer, and when they do answer they just transfer you to the correct agency. In my city, cell phone calls go directly to the jurisdiction that the cell tower is in, so it works a lot better.
We answer 911 as well as the 7 digit non-emergency line. Its funny that some people overuse 911, like for a cat stuck in a tree.... and then some people are afraid of 911 so they call the non-emergency number for a felony in progress or life threatening problem.
Most of the stuff people have mentioned are very 911 appropriate.