I've searched around and didn't find anything on this.
I went to T-Mobile today because my data signal seemed to be getting worse. They swapped out my son card and that caused me to have a voicemail notification. I called voicemail but there's nothing in it. Now I can't get the icon to go away. I'll admit it, I'm a little ocd about icons HAVING to be cleared of notifications and its driving me insane.
I did this just before reading this post. I had to deactivate my google voice and reactivate it afterwards to fix it. What a pain in the butt. Now my visual voicemail won't turn on. I was gonna run without google voice for a couple days. Worked before I traded out sim cards but now nothing. Oh well, guess I'll have to wait for the real deal mango build to come out.