I ran into this same problem today. At first, it gave me the option to Cancel or Upgrade. After hitting Cancel, when I closed it and went back to Voicemail, it only gave me the "option" to Upgrade, and disabled the Back button on the phone. It could not disable the Home button, though I am sure they would have liked to.
I went into Settings -> More -> Apps -> All and selected Voicemail. From there, I clicked on the Uninstall Updates (it might have been Remove Updates). After that, when I went into Voicemail again, it prompted me to Cancel or Update, so I chose Cancel. I went back in and it no longer prompted me to update. So this solved the problem for now.
I am pretty sure that this is a temporary fix, though. It may push another update which forces me into that same "no option" situation again. We'll see!