See...I tried that, and it didn't work for me, but one of the things I tried was calling someone with the headset plugged in. No sound through the headphones, but when I would mute and unmute, I would hear the caller through the headphones, but they wouldn't be able to hear me.
I'm going to try your method and actually shut down my phone and restart it. I will try it with my voicemail and see what the results are.
EDIT: Tried it, and all it did was go into speaker mode with the headphones plugged in. I was able to mute and unmute leaving myself 4 google voice messages. The only successful test was the speakerphone method.