Recently I've gotten a significant number of ridiculous voicemails. I wish I could share them with you but for privacy and respect for the callers, I won't. What I will do is to give you some very simple advice. I hope you follow it. I received this same advice early in my career from a senior executive; some of the best advice I've gotten in regards to effective communications methods.
1. Keep it short. 45 seconds is enough to get your point across. A minute is acceptable. 1 and 1/2 minutes is really testing our patience. You hit the 2 minute mark and your name is beginning to get written in the black list. If you hit 3 minutes or longer you will mostly be ignored. A voicemail is an archaic device that was intended to leave a quick announcement or a request for a call back. TXT/SMS serves much better as a quick announcement system, and a missed call usually will prompt a call-back.
2. Building up on #1. Avoid rambling on; don't tell a story, don't brainstorm, don't pitch, just get to the point of what you need to say starting at the beep.
3. Keep it in point. Do not leave a voicemail if you're doing something else. Not only is it rude and disrespectful, it is ineffective and at the end of your voicemail you will have wasted 2 - 3 minutes of your day and the other person's day. If you know David Allen's philosophy of GTD, 2 minutes can make or break your day.
4. Leave your name and phone number, clearly. Twice. Once at the beginning, once at the end. I do it twice at the end so that the person listening has a chance to write it down, or remember it easily. This isn't so important if the person already knows you, but if they don't, you risk being ignored or lost in the shuffle.
I could explain more, I could rant, bitch and moan about all the wasted time listening to voicemails, knowing quite well I'm just going to call you back and you're going to repeat all of it. I hope you know that this advice is golden, and I think these 4 points should be sufficient. Don't you agree with me that long voicemails suck?