Uh-huh. So those things really DO have minds of their own . . .
Oh and as for the OP, I second the “Bravo!” That is a damn good story! Never happened to me, but then my sister and I do voicemail Karaoke to each other on purpose (usually just snippets, not whole songs, and certainly not entire repertoires). These days, what with her 3-year-old twins and all, her snippets are usually Sesame Street classics. I, on the other hand, have been stifling an urge to call her machine and sing,
*I’ve had a hoot-owl howlin’ by my window now . . .
fer SIX NIIIIIGHTS in a RO-O-OWWWW . . . *
Any story with a reference to Dance Party USA is hilarious! 
I keep my cell phone in my purse and I’m always paranoid about singing along with the car radio after accidentally speed dialing someone. The keyguard is usually turned on, but you never know. Now I’m more afraid than ever that it will happen one of these days. And I would never qualify for American Idol (not just because I’m too old).
Two stories:
A friend of mine was at a strip club attending a Bachelor party. He had his cellphone in his back pocket. Apparently several times as he stood up/sat down, he hit the redial key on his phone. Unfortunately, the last number he had called was the bride’s parent’s house - the groom had needed to speak to his soon-to-be mother-in-law much earlier in the evening. So four or five times throughout the night the m.i.l. answered her phone to hear the cheering, yelling, whooping etc as the groom was “entertained” by the strippers.
When I was buying a house, I called a bunch of home inspectors to find one I wanted to use. I left a voicemail for one, and he called me back about 20 minutes later. I was on the phone, so he went to my voicemail. After leaving a nice message, he must have neglected to turn off his phone and then started cursing me out for wasting his time, leaving a number that I didn’t answer directly, being a jerk for using voicemail, etc. I called him back, then played his voicemail back for him, then told him I wouldn’t be needing his services.